config: use mini-os master for unstable

config: use mini-os master for unstable

We haven't used mini-os master for about 2 years now due to a stubdom
test failing [1]. Booting a guest with mini-os master used for building
stubdom didn't reveal any problem, so use master for unstable in order
to let OSStest find any problems not showing up in the local test.

[1]: https://lists.xen.org/archives/html/minios-devel/2018-04/msg00015.html

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>