virt: replace status string with constant

during the extraction of migration.py some status strings
survived to the port to vmstatus. This patch fixes this.

Change-Id: Ic3bc8289141622561b0ce494598c7f7af1d6f837
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29110
Reviewed-by: Dan Kenigsberg <danken@redhat.com>