Some monitor commands may take a very long time to complete. It is
not desirable to block other incoming API calls forever. With this
change, if an existing API call is holding the job lock, additional
API calls will not wait forever. They will time out after a short
period of time, allowing application to retry later.

* include/libvirt/virterror.h, src/util/virterror.c: Add new
  VIR_ERR_OPERATION_TIMEOUT error code
* src/qemu/qemu_driver.c: Change to a timed condition variable
  wait for acquiring the monitor job lock