storage: resize vol against real allocated size

storage: resize vol against real allocated size

Currently, 'vol-resize --allocate' allocates new space at the
vol->capacity offset. But the vol->capacity is not necessarily the same
as vol->allocation. For instance:.

	[root@localhost ~]# virsh vol-list --pool tmp-pool --details
	 Name      Path                   Type  Capacity  Allocation
	-------------------------------------------------------------
	 tmp-vol  /root/tmp-pool/tmp-vol  file  1.00 GiB  1.00 GiB

	[root@localhost ~]# virsh vol-resize tmp-vol --pool tmp-pool 2G

	[root@localhost ~]# virsh vol-list --pool tmp-pool --details
	 Name      Path                   Type  Capacity  Allocation
	-------------------------------------------------------------
	 tmp-vol  /root/tmp-pool/tmp-vol  file  2.00 GiB  1.00 GiB

So, if we want to allocate more bytes, so the file is say 3G big, the
real allocated size is 2G actually:

	[root@localhost ~]# virsh vol-resize tmp-vol --pool tmp-pool 3G --allocate

	[root@localhost ~]# virsh vol-list --pool tmp-pool --details
	 Name      Path                   Type  Capacity  Allocation
	-------------------------------------------------------------
	 tmp-vol  /root/tmp-pool/tmp-vol  file  3.00 GiB  2.00 GiB

This commit uses the correct vol->allocation instead of incorrect
vol->capacity, so the output of the commands above looks like this:

	[root@localhost ~]# virsh vol-resize tmp-vol --pool tmp-pool 3G --allocate

	[root@localhost ~]# virsh vol-list --pool tmp-pool --details
	 Name      Path                   Type  Capacity  Allocation
	-------------------------------------------------------------
	 tmp-vol  /root/tmp-pool/tmp-vol  file  3.00 GiB  3.00 GiB

Moreover, if the '--alocate' flag was used, we must update the
vol->allocation member in storageVolResize API too, not just
vol->capacity.

Reported-by: Wang Sen <wangsen@linux.vnet.ibm.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>