x86/p2m: pass the p2m to write_p2m_entry handlers

x86/p2m: pass the p2m to write_p2m_entry handlers

Current callers pass the p2m to paging_write_p2m_entry, but the
implementation specific handlers of the write_p2m_entry hook instead
of a p2m get a domain struct due to the handling done in
paging_write_p2m_entry.

Change the code so that the implementations of write_p2m_entry take a
p2m instead of a domain.

This is a non-functional change, but will be used by follow up
patches.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>