qemu-trad: xenstore: use relative path for device-model node

qemu-trad: xenstore: use relative path for device-model node

For QEMU traditional running in Dom0, there is no functional change
because it will still write to the same /local/domain/0 path.

For QEMU traditional stubdom, this is an incompatible startup
protocol change.  There is a corresponding libxl changeset "libxl:
use new QEMU xenstore protocol", which has not yet been committed
(54c2e621 was reverted in 84066dd4).

QEMU traditional stubdom was broken by a0731cca "ioreq-server: on-demand
creation of ioreq server" in 4.5. Currently there is a workaround in
-unstable dd748d12 "x86/hvm: wait for at least one ioreq server to be
enabled" (which should be backported to 4.5). QEMU traditional stubdom
works with that workaround in -unstable but it's not ideal situation.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>