migration: Fail migration blocker for --only-migratable

migration: Fail migration blocker for --only-migratable

migrate_add_blocker should rightly fail if the '--only-migratable'
option was specified and the device in use should not be able to
perform the action which results in an unmigratable VM.

Make migrate_add_blocker return -EACCES in this case.

Signed-off-by: Ashijeet Acharya <[email protected]>
Message-Id: <1484566314[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>