When adding domainMemoryStats API support for the qemu driver, I didn't
follow the locking rules exactly.  The job condition must be held when
executing monitor commands.  This corrects the segfaults I was seeing
when calling domainMemoryStats in a multi-threaded environment.

* src/qemu/qemu_driver.c: in qemudDomainMemoryStats() add missing
  calls to qemuDomainObjBeginJob/qemuDomainObjEndJob