virt: Add information about 'source' for block device

virt: Add information about 'source' for block device

This patch adds support for checking if source property for
block device exists - this information is stored as
a 'has_source' boolean field in struct block_device.

Checking if source for block device exists is needed in scenario
when 'block info' is fetched from libvirt. 'Block info' can be fetched
only for block devices with source defined - otherwise libvirt throws
errors.

Change-Id: I76aeaf140f45f58178669e8a2a1f4d3fe980d658
Signed-off-by: Radoslaw Jablonski <radoslawx.jablonski@intel.com>