x86/PV: assert page state in mark_pv_pt_pages_rdonly()

x86/PV: assert page state in mark_pv_pt_pages_rdonly()

About every time I look at dom0_construct_pv()'s "calculation" of
nr_pt_pages I question (myself) whether the result is precise or merely
an upper bound. I think it is meant to be precise, but I think we would
be better off having some checking in place. Hence add ASSERT()s to
verify that
- all pages have a valid L1...Ln (currently L4) page table type and
- no other bits are set, in particular the type refcount is still zero.

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