core: cleanup_vifs should not fail when no network intf is present
core: cleanup_vifs should not fail when no network intf is present

This can happen when initially there was no default netvm, some domain
was started, then default netvm was set and started - then
netvm.connected_vms will contain domains which aren't really connected
there.
Especially this was happening in firstboot.

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