x86/pv: Move segment descriptor infrastructure into PV-only files

x86/pv: Move segment descriptor infrastructure into PV-only files

... so all segment checking/adjustment logic is co-located.

Perform some trivial style cleanup to check_descriptor() as it moves,
converting types, and cleaning up trailing whitespace.

In particular, this means that check_descriptor() is now excluded from
!CONFIG_PV builds.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>