Implements domainMemStats in the qemu driver

Implements domainMemStats in the qemu driver

Support for memory statistics reporting is accepted for qemu inclusion.
Statistics are reported via the monitor command 'info balloon' as a comma
seprated list:

(qemu) info balloon
balloon: actual=1024,mem_swapped_in=0,mem_swapped_out=0,major_page_faults=88,minor_page_faults=105535,free_mem=1017065472,total_mem=1045229568

Libvirt, qemu, and the guest operating system may support a subset of the
statistics defined by the virtio spec.  Thus, only statistics recognized by
components will be reported.

* src/qemu/qemu_driver.c src/qemu/qemu_monitor_text.[ch]: implement the
  new entry point by using info balloon monitor command