Add virDomainGetBlockInfo API to query disk sizing

Add virDomainGetBlockInfo API to query disk sizing

Some applications need to be able to query a guest's disk info,
even for paths not managed by the storage pool APIs. This adds
a very simple API to get this information, modelled on the
virStorageVolGetInfo API, but with an extra field 'physical'.
Normally 'physical' and 'allocation' will be identical, but
in the case of a qcow2-like file stored inside a block device
'physical' will give the block device size, while 'allocation'
will give the qcow2 image size

* include/libvirt/libvirt.h.in: Define virDomainGetBlockInfo