virDomainBlockStats(Flags): Produce saner error message on empty disk path

virDomainBlockStats(Flags): Produce saner error message on empty disk path

As of 0bd2ccdec an empty disk path for virDomainBlockStats (or the one
with Flags) is allowed meaning "get me overall summarized statistics".
However, running 'virsh domblkstat $dom' throws a misleading error:

  # ./tools/virsh domblkstat dom
  error: Failed to get block stats dom
  error: invalid argument: invalid path:

while after this commit

  # virsh domblkstat dom
  error: Operation not supported: summary statistics are not supported yet

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>