x86/P2M: p2m_change_type() should pass on error from p2m_set_entry()

x86/P2M: p2m_change_type() should pass on error from p2m_set_entry()

Modify the function's name to help eventual backports involving this
function, and in one case where this is trivially possible also stop
ignoring its return value.

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