x86: limit page type width

x86: limit page type width

There's no reason to burn 4 bits on page type when we only have 7 types
(plus "none") at present. This requires changing one use of
PGT_shared_page, which so far assumed that the type is both a power of
2 and the only type with the high bit set.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>