For work I run a personal and a work VM on my laptop. When I was at VMware I dogfooded internal builds of Workstation which worked well, but was always a challenge to have its additions consistently building against latest kernels. About 5 and half years ago, the only practical alternative option was VirtualBox. IIRC SPICE maybe didn't even exist or was very early, and while VNC is OK to fiddle with something, completely impractical for primary daily use.
VirtualBox is fine, but there is the promised land of all the great features of qemu/kvm and many recent improvements in 3D integration always calling. I'm trying all this on my Fedora 28 host, with a Fedora 28 guest (which has been in-place upgraded since Fedora 19), so everything is pretty recent. Periodically I try this conversion again, but, spoiler alert, have not yet managed to get things quite right.
As I happened to close an IRC window, somehow my client seemed to crash X11. How odd ... so I thought, everything has just disappeared anyway; I might as well try switching again.
Image conversion has become much easier. My primary VM has a number of snapshots, so I used the VirtualBox GUI to clone the VM and followed the prompts to create the clone with squashed snapshots. Then simply convert the VDI to a RAW image with
$ qemu-img convert -p -f vdi -O raw image.vdi image.raw
Note if you forget the progress meter, send the pid a SIGUSR1 to get it to spit out a progress.
virt-manager has come a long way too. Creating a new VM was trivial. I wanted to make sure I was using all the latest SPICE gl etc., stuff. Here I hit some problems with what seemed to be permission denials on drm devices before even getting the machine started. Something suggested using libvirt in session mode, with the qemu:///session URL -- which seemed more like what I want anyway (a VM for only my user). I tried that, put the converted raw image in my home directory and the VM would boot. Yay!
It was a bit much to expect it to work straight away; while GRUB did start, it couldn't find the root disks. In hindsight, you should probably generate a non-host specific initramfs before converting the disk, so that it has a larger selection of drivers to find the boot devices (especially the modern virtio drivers). On Fedora that would be something like
sudo dracut --no-hostonly --regenerate-all -f
As it turned out, I "simply" attached a live-cd and booted into that, then chrooted into my old VM and regenerated the initramfs for the latest kernel manually. After this the system could find the LVM volumes in the image and would boot.
After a fiddly start, I was hopeful. The guest kernel dmesg DRM sections showed everything was looking good for 3D support, along with the glxinfo showing all the virtio-gpu stuff looking correct. However, I could not get what I hoped was trivial automatic window resizing happening no matter what. After a bunch of searching, ensuring my agents were running correctly, etc. it turns out that has to be implemented by the window-manager now, and it is not supported by my preferred XFCE (see https://bugzilla.redhat.com/show_bug.cgi?id=1290586). Note you can do this manually with xrandr --output Virtual-1 --auto to get it to resize, but that's rather annoying.
I thought that it is 2018 and I could live with Gnome, so installed that. Then I tried to ping something, and got another selinux denial (on the host) from qemu-system-x86 creating icmp_socket. I am guessing this has to do with the interaction between libvirt session mode and the usermode networking device (filed https://bugzilla.redhat.com/show_bug.cgi?id=1609142). I figured I'd limp along with ICMP and look into details later...
Finally when I moved the window to my portrait-mode external monitor, the SPICE window expanded but the internal VM resolution would not expand to the full height. It looked like it was taking the height from the portrait-orientation width.
Unfortunately, forced swapping of environments and still having two/three non-trivial bugs to investigate exceeded my practical time to fiddle around with all this. I'll stick with VirtualBox for a little longer; 2020 might be the year!