This new capability enables a pause before device state serialization so
that we can finish all block jobs without racing with the end of the
migration. The pause is indicated by "pre-switchover" state. Once we're
done QEMU enters "device" migration state.

This patch just defines the new capability and QEMU migration states and
their mapping to our job states.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>