qemu-iotests: add tests for runtime fd passing via SCM rights

This case will test whether the monitor can receive fd at runtime.
To verify better, additional monitor is created to see if qemu
can handler two monitor instances correctly.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>