virt: migration: restore the usage of 'vmstatus'

the usage of the 'vmstatus' constants was lost on
destination in the split of the migration.py.

Change-Id: I181a728a0b75ce0982a52cae27b0c366208a6cf8
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27051
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>