x86/shadow: encode full GFN in magic MMIO entries

x86/shadow: encode full GFN in magic MMIO entries

Since we don't need to encode all of the PTE flags, we have enough bits
in the shadow entry to store the full GFN. Limit use of literal numbers
a little and instead derive some of the involved values. Sanity-check
the result via BUILD_BUG_ON()s.

This then allows dropping from sh_l1e_mmio() again the guarding against
too large GFNs. It needs replacing by an L1TF safety check though, which
in turn requires exposing cpu_has_bug_l1tf.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>