monitor: Use QEMU_CLOCK_VIRTUAL for the event queue in qtest mode

This allows us to perform tests on the monitor queues to verify that
the rate limits are enforced.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: dde511809e954a5c32d5b648bb184c03c89ed5d5.1457610443.git.berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
