x86/pv: Consistently use typesafe helpers in all files

x86/pv: Consistently use typesafe helpers in all files

Rather than having a mix of code behaviour.  This requires updating
pagetable_{get,from}_page() to use the non-overridden helpers.

This requires some adjustments in priv_op_{read,write}_cr(), which is most
easily done by switching CR3 handling to using mfn_t.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>