xen/bitops: Convert 'hweight(x) > 1' to new multiple_bits_set()

xen/bitops: Convert 'hweight(x) > 1' to new multiple_bits_set()

Using hweight() is an especially expensive way of determining simply if
multiple bits are set in a value.  Worse, 4 of the 10 hweight() calls in Xen
are of this form.

Switch to the new multiple_bits_set() helper.  This is far more efficient than
the longhand hweight() algorithm and, owing to its simplicity, likely more
efficient than even a dedicated instruction on a superscalar processor.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>