qemu-nbd: Add -qmp option to qemu-nbd.

This patch add a -qmp parameter to the qemu-nbd command line in order to be able
to manipulate the qemu-nbd block devices via qmp.

This could be used with containers to easily migrate from qemy/kvm to a container.

Signed-off-by: Benoit Canet <benoit@irqsave.net>