[XEN] Fix compatibility with future guests which may try to

[XEN] Fix compatibility with future guests which may try to
use hypercalls >= NR_hypercalls. These must fail with ENOSYS,
but the current strategy of masking off the high-order bits of
the hypercall number means we instead map those hypercalls onto
lower-numbered hypercalls with unpredictable results. This patch
replaces masking with an explicit compare-and-jump.
Signed-off-by: Keir Fraser <keir@xensource.com>