qemu: Always set migration capabilities

qemu: Always set migration capabilities

We used to set migration capabilities only when a user asked for them in
flags. This is fine when migration succeeds since the QEMU process is
killed in the end but in case migration fails or if it's cancelled, some
capabilities may remain turned on with no way to turn them off. To fix
that, migration capabilities have to be turned on if requested but
explicitly turned off in case they were not requested but QEMU supports
them.

https://bugzilla.redhat.com/show_bug.cgi?id=1163953
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>