minios: use more recent commit for building xen unstable

minios: use more recent commit for building xen unstable

Commit 82c3d15c903aa43 ("minios: Revert recent change and revert to
working minios") switched the used commit for the build of Xen unstable
from master to a rather old commit (the one used for Xen 4.13 instead
of the last one without a known problem).

Switch to Mini-OS commit 051b87bb9c196 instead, which doesn't contain
the problematic modification being reason for switching away from
master.

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