xen/page_alloc: introduce preserved page flags macro

xen/page_alloc: introduce preserved page flags macro

PGC_static and PGC_extra needs to be preserved when assigning a page.
Define a new macro that groups those flags and use it instead of or'ing
every time.

To make preserved flags even more meaningful, they are kept also when
switching state in mark_page_free().

Signed-off-by: Carlo Nonato <carlo.nonato@minervasys.tech>
Reviewed-by: Jan Beulich <jbeulich@suse.com>