x86/vpic: Improve bitops usage

x86/vpic: Improve bitops usage

 * For vpic_get_priority(), introduce a common ror8() helper in plain C.  One
   thing that I can't persuade the compiler to realise is that a non-zero
   value rotated is still non-zero, so use __builtin_clz() to help the
   optimiser out.

 * vpic_ioport_write() can be simplified to just for_each_set_bit(), which
   avoids spilling pending to the stack each loop iteration.  Changing pending
   from unsigned int to uint8_t isn't even strictly necessary given the
   underlying types of vpic->isr and vpic->irr, but done so clarity.

No functional change.

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