Allow the monitor to be suspended during non-blocking op

Allow the monitor to be suspended during non-blocking op

Live migration happens in the background, but it is useful to make the monitor
command appear as if it's blocking.  This allows a management tool to
immediately know when the live migration has completed without having to poll
the migration status.

This patch allows the monitor to be suspended from a monitor callback which
will prevent new monitor commands from being executed.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5431 c046a42c-6fe2-441c-8c8c-71466251a162