virt: Add block info statistics for block devices

virt: Add block info statistics for block devices

virDomainBlockInfo statistics are now exposed through virt
plugin as:
-disk_allocation
-disk_capacity
-disk_physical

Those statistics are optional. They can be enabled by adding
corresponding entries to ExtraStats section in virt plugin
configuration in collectd.conf.

New ExtraStats are:
-'disk_allocation'
-'disk_capacity'
-'disk_physical'

Change-Id: I75475eca8b9eb402a894e2ae3c70c8b09b0eed9a
Signed-off-by: Radoslaw Jablonski <radoslawx.jablonski@intel.com>