ssh: Don't crash if either host or path is not specified.

$ ./qemu-img create -f qcow2 overlay \
    -b 'json: { "file.driver":"ssh",
                "file.host":"localhost",
                "file.host_key_check":"no" }'
qemu-img: qobject/qdict.c:193: qdict_get_obj: Assertion `obj != ((void *)0)' failed.
Aborted

A similar crash also happens if the file.host field is omitted.

https://bugzilla.redhat.com/show_bug.cgi?id=1147343

Bug found and reported by Jun Li.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
