migration: add properties and string constants

This patch adds constants for state and method and helper properties
to avoid the client code to mess up with the migration.SourceThread
internals, and to make the migration code looks a bit nicer.

Change-Id: I2bfbc71e25dddf623ad2d79d5ee66f1501dbb81b
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29111
Reviewed-by: Dan Kenigsberg <danken@redhat.com>