virnuma: Introduce pages helpers

virnuma: Introduce pages helpers

For future work we need two functions that fetches total number of
pages and number of free pages for given NUMA node and page size
(virNumaGetPageInfo()).

Then we need to learn pages of what sizes are supported on given node
(virNumaGetPages()).

Note that system page size is disabled at the moment as there's one
issue connected. If you have a NUMA node with huge pages allocated the
kernel would return the normal size of memory for that node. It
basically ignores the fact that huge pages steal size from the system
memory. Until we resolve this, it's safer to not confuse users and
hence not report any system pages yet.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>