virsh has to be able to work remotely so this code will definitely
not
be acceptable here.
I think we can only go with adding a "type" field for the diskspec
string which will be either "file" or "block" and do this in that
case.
e.g.
--diskspec vda,snapshot=external,driver=qcow2,type=block,file=/dev/whatever
thanks, i will reconsider adding a "type" field for --diskspec to amend this
patch.