Improve runtime detection of KVM support

[simondeziel]

Due to suspected issue with kvm module being loaded and crash during suspend, I rmmod kvm before suspending my laptop. This causes LXD to not noticing that KVM is still supported:

$ lxc launch ubuntu-minimal-daily:24.04 v1 --vm -p lxd-ci
Launching v1
Error: Failed instance creation: Failed creating instance record: Instance type "virtual-machine" is not supported on this server: KVM support is missing (no /dev/kvm)

# Fair, the modules were not loaded, let's fix it
$ sudo modprobe kvm kvm_amd

$ lxc launch ubuntu-minimal-daily:24.04 v1 --vm -p lxd-ci
Launching v1
Error: Failed instance creation: Failed creating instance record: Instance type "virtual-machine" is not supported on this server: KVM support is missing (no /dev/kvm)

This second attempt should have worked but did not.

Workaround: restart LXD for it to pick up the KVM support:

$ sudo snap restart lxd
2024-11-09T11:04:58-05:00 INFO Waiting for "snap.lxd.daemon.service" to stop.
Restarted.
$ lxc launch ubuntu-minimal-daily:24.04 v1 --vm -p lxd-ci
Launching v1

Additional information:

$ snap list lxd
Name  Version      Rev    Tracking     Publisher   Notes
lxd   git-5c0f573  31064  latest/edge  canonical✓  -