x86/mm: print domain IDs instead of pointers

x86/mm: print domain IDs instead of pointers

Printing pointers to struct domain isn't really useful for initial
problem analysis. In get_page() also drop the page only after issuing
the log message, so that at the time of printing the state can be
considered reasonably consistent.

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