Query block allocation extent from QEMU monitor

Query block allocation extent from QEMU monitor

The virDomainGetBlockInfo API allows query physical block
extent and allocated block extent. These are normally the
same value unless storing a special format like qcow2
inside a block device. In this scenario we can query QEMU
to get the actual allocated extent.

Since last time:

 - Return fatal error in text monitor
 - Only invoke monitor command for block devices
 - Fix error handling JSON code

* src/qemu/qemu_driver.c: Fill in block aloction extent when VM
  is running
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h,
  src/qemu/qemu_monitor_json.c, src/qemu/qemu_monitor_json.h,
  src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h: Add
  API to query the highest block extent via info blockstats