guest_physmap_remove_page() needs its return value checked

guest_physmap_remove_page() needs its return value checked

Callers, namely such subsequently freeing the page, must not blindly
assume success - the function may namely fail when needing to shatter a
super page, but there not being memory available for the then needed
intermediate page table.

As it happens, guest_remove_page() callers now also all check the
return value.

Furthermore a missed put_gfn() on an error path in gnttab_transfer() is
also being taken care of.

This is part of XSA-222.

Reported-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>