qemu: bulk stats: add block allocation information

qemu: bulk stats: add block allocation information

Management software wants to be able to allocate disk space on demand.
To support this they need keep track of the space occupation of the
block device.  This information is reported by qemu as part of block
stats.

This patch extend the block information in the bulk stats with the
allocation information.

To keep the same behaviour a helper is extracted from
qemuMonitorJSONGetBlockExtent in order to get per-device allocation
information.

Signed-off-by: Francesco Romani <fromani@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>