block: fix handling non-dom0 backend
block: fix handling non-dom0 backend

The libvirt XML config syntax was changed - the element is named
<backenddomain/>.

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