[XEN] Scheduling hypercalls defer entry to the scheduler to softirq

[XEN] Scheduling hypercalls defer entry to the scheduler to softirq
context.

This avoids entering the scheduler with outstanding multicall state
and also happens to simplify the x86 entry protocol to the scheduler
hypercall (since we do not need to preset the return code).

Signed-off-by: Keir Fraser <keir@xensource.com>