qemu: Implement bulk stats API and one of the stats groups to return

qemu: Implement bulk stats API and one of the stats groups to return

Implement the API function for virDomainListGetStats and
virConnectGetAllDomainStats in a modular way and implement the
VIR_DOMAIN_STATS_STATE group of statistics.

Although it may look like the function looks universal I'd rather not
expose it to other drivers as the coming stats groups are likely to do
qemu specific stuff to obtain the stats.