qemuMigrationDstPrepareAny: Parse cookie before adding domain onto list

qemuMigrationDstPrepareAny: Parse cookie before adding domain onto list

There are some checks done when parsing a migration cookie. For
instance, one of the checks ensures that the domain is not being
migrated onto the same host. If that is the case, then we are in
big trouble because the @vm is the same domain object used by
source and it has some jobs sets and everything so recovering
from failed cookie parsing would be needlessly hard.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>