virt: migration: rename migration threads

drop the now redundant 'Migration' prefix.
No functional or behavioural changes.

Change-Id: I47597b8a0e89d093e0ff725518a80d3ec6de2cfe
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26260
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>