virt: migration: replace magic number

replace a magic constant with the symbolic name to clarify
the intent of the code.

What the code does is to check the migration job is still active.
The symbolic name was found by checking the docs:

http://libvirt.org/html/libvirt-libvirt.html#virDomainJobInfo

(beware of misalinged comments).
'jobType' can be one of
http://libvirt.org/html/libvirt-libvirt.html#virDomainJobType

Change-Id: I7ef125866ed7d1590f1c0cee0084daf86c6ee912
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28356
Reviewed-by: Dan Kenigsberg <danken@redhat.com>