xenpaging: move page add/resume loops into its own function.

xenpaging: move page add/resume loops into its own function.

Move page resume loop into its own function.
Move page eviction loop into its own function.
Allocate all possible slots in a paging file to allow growing and
shrinking of the number of paged-out pages. Adjust other places to
iterate over all slots.

This change is required by subsequent patches.

v2:
 - check if victims allocation succeeded

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson.citrix.com>