qubes/tests: do not undefine libvirt domain twice
qubes/tests: do not undefine libvirt domain twice

Do not access vm.libvirt_domain after it being already removed - this
will redefine it again in libvirt, just to undefine it in a moment.
On the other hand, few lines below there is fallback libvirt cleanup, in
case of proper one not working.

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