x86: drop high compat r/o M2P table address range

x86: drop high compat r/o M2P table address range

Now that we don't properly hook things up into the page tables anymore
we also don't need to set aside an address range. Drop it, using
compat_idle_pg_table_l2[] simply (explicitly) from slot 0.

While doing the re-arrangement, which is accompanied by the dropping or
replacing of some local variables, restrict the scopes of some further
ones at the same time.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wl@xen.org>