If QEMU supports the BALLOON_EVENT QMP event, then we can
avoid invoking 'query-balloon' when returning XML or the
domain info.

* src/qemu/qemu_capabilities.c, src/qemu/qemu_capabilities.h:
  Add QEMU_CAPS_BALLOON_EVENT
* src/qemu/qemu_driver.c: Skip query-balloon in
  qemudDomainGetInfo and qemuDomainGetXMLDesc if we have
  QEMU_CAPS_BALLOON_EVENT set
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Check
  for BALLOON_EVENT at connect to monitor. Add callback
  for balloon change notifications
* src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h:
  Add handling of BALLOON_EVENT and impl 'query-events'
  check
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>