https://bugzilla.redhat.com/show_bug.cgi?id=1206521

If the backend driver updates the pool available and/or allocation values,
then the storage_driver VolCreateXML, VolCreateXMLFrom, and VolDelete APIs
should not change the value; otherwise, it will appear as if the values
were "doubled" for each change.  Additionally since unsigned arithmetic will
be used depending on the size and operation, either or both values could be
appear to be much larger than they should be (in the EiB range).

Currently only the disk pool updates the values, but other pools could.
Assume a "fresh" disk pool of 500 MiB using /dev/sde:

$ virsh pool-info disk-pool
...
Capacity:       509.88 MiB
Allocation:     0.00 B
Available:      509.84 MiB

$ virsh vol-create-as disk-pool sde1 --capacity 300M

$ virsh pool-info disk-pool
...
Capacity:       509.88 MiB
Allocation:     600.47 MiB
Available:      16.00 EiB

Following assumes disk backend updated to refresh the disk pool at deletion
of primary partition as well as extended partition:

$ virsh vol-delete --pool disk-pool sde1
Vol sde1 deleted

$ virsh pool-info disk-pool
...
Capacity:       509.88 MiB
Allocation:     9.73 EiB
Available:      6.27 EiB

This patch will check if the backend updated the pool values and honor that
update.