x86+libxl: correct p2m (shadow) memory pool size calculation

x86+libxl: correct p2m (shadow) memory pool size calculation

The reference "to shadow the resident processes" is applicable to
domains (potentially) running in shadow mode only. Adjust the
calculations accordingly. This, however, requires further parameters.
Since the original function is deprecated anyway, and since it can't be
changed (for being part of a stable ABI), introduce a new (internal
only) function, with the deprecated one simply becoming a wrapper.

In dom0_paging_pages() also take the opportunity and stop open-coding
DIV_ROUND_UP().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>