x86/p2m: Allow p2m_get_page_from_gfn to return shared entries

x86/p2m: Allow p2m_get_page_from_gfn to return shared entries

The owner domain of shared pages is dom_cow, use that for get_page
otherwise the function fails to return the correct page under some
situations. The check if dom_cow should be used was only performed in
a subset of use-cases. Fixing the error and simplifying the existing check
since we can't have any shared entries with dom_cow being NULL.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@intel.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>