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 to a bool return type, 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

CC: Jan Beulich JBeulich@suse.com CC: Roger Pau Monné roger.pau@citrix.com CC: Wei Liu wl@xen.org