The change 18c2a592 caused
some regressions in behaviour of virDomainBlockStats
and virDomainBlockInfo in the QEMU driver.

The virDomainBlockInfo API stopped working for inactive
guests if querying a block device.

The virDomainBlockStats API did not promptly report
an error if the guest was not running in some cases.

* src/qemu/qemu_driver.c: Fix inactive guest handling
  in BlockStats/Info APIs