x86/P2M: simplify write_p2m_entry()

x86/P2M: simplify write_p2m_entry()

The "table_mfn" parameter really isn't needed anywhere, so it gets
dropped.

The "struct vcpu *" one was always bogus (as was being made up by
paging_write_p2m_entry()), and is not commonly used. It can be easily
enough made up in the one place (sh_unshadow_for_p2m_change()) it is
needed, and we can otherwise pass "struct domain *" instead, properly
reflecting that P2M operations are per-domain.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>