vring: avoid expensive divides

Replace divides by variables and by the hard constants with masks and
divides by easy constants.

Improves netperf by about 1.6%.

Noted by Vlad.

Reviewed-by: Dor Laor <dor@cloudius-systems.com>
Reviewed-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>