Implement the qemu driver's virDomainQemuMonitorCommand
and hook it into the API entry point.

Changes since v1:
 - Rename the (external) qemuMonitorCommand to qemuDomainMonitorCommand
 - Add virCheckFlags to qemuDomainMonitorCommand

Changes since v2:
 - Drop ATTRIBUTE_UNUSED from the flags

Changes since v3:
 - Add a flag to priv so we only print out monitor command warning once.  Note
   that this has not been plumbed into qemuDomainObjPrivateXMLFormat or
   qemuDomainObjPrivateXMLParse, which means that if you run a monitor command,
   restart libvirtd, and then run another monitor command, you may get an
   an erroneous VIR_INFO.  It's a pretty minor matter, and I didn't think it
   warranted the additional code.
 - Add BeginJob/EndJob calls around EnterMonitor/ExitMonitor
Signed-off-by: NChris Lalancette <clalance@redhat.com>