The gluster protocol in qemu uses two styles, one of which is legacy and
not covered by the QAPI schema.

To allow using of the new style in the blockdev-add code, add a
parameter for qemuBlockStorageSourceGetBackendProps which will switch
between the two modes.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>