qubes: don't fail VM removal if already undefined in libvirt
qubes: don't fail VM removal if already undefined in libvirt

This may happen when it was manually undefined for some reason. Or
during tests cleanup (tearDown).

woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin