x86/PV: polish pv_set_gdt()

x86/PV: polish pv_set_gdt()

There's no need to invoke get_page_from_gfn(), and there's also no need
to update the passed in frames[]. Invoke get_page_and_type() directly.

Also make the function's frames[] parameter const, change its return
type to int, and drop the bogus casts from two of its invocations.

Finally a little bit of cosmetics.

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