qemu: Pass through arguments of 'ssh' block driver used by libguestfs

We currently don't model the 'ssh' protocol properties properly and
since it seems impossible for now (agent path passed via environment
variable). To allow libguestfs to work as it used in pre-blockdev era we
must carry the properties over to the command line. For this instance we
just store it internally and format it back.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>