core: support pin-to-any

Fixing a regression that forces to specify a host
for non-migratable VMs.

Change-Id: I10d60bb9db2506a63a19f5e75446ddac32b8dc3e
Bug-Url: https://bugzilla.redhat.com/1021574
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>