x86/P2M: consolidate handling of types not requiring a valid MFN

x86/P2M: consolidate handling of types not requiring a valid MFN

As noted regarding the mixture of checks in p2m_pt_set_entry(),
introduce a new P2M type group allowing to be used everywhere we
just care about accepting operations with either a valid MFN or a type
permitting to be used without (valid) MFN.

Note that p2m_mmio_dm is not included in P2M_NO_MFN_TYPES, as for the
intended purpose that one ought to be treated similar to p2m_invalid
(perhaps the two should ultimately get folded anyway).

Note further that PoD superpages now get INVALID_MFN used when creating
page table entries (was _mfn(0) before).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>