xen/arm: Trap and yield on WFE instructions

xen/arm: Trap and yield on WFE instructions

If we have a Guest/DomU with two or more of its VCPUs running
on same host CPU then it can quite likely happen that these
VCPUs fight for same spinlock and one of them will waste CPU
cycles in WFE instruction. This patch makes WFE instruction
trap for VCPU and forces VCPU to yield its timeslice.

The KVM ARM/ARM64 also does similar thing for handling WFE
instructions. (Please refer,
https://lists.cs.columbia.edu/pipermail/kvmarm/2013-November/006259.html)

In general, this patch is more of an optimization for an
oversubscribed system having number of VCPUs more than
underlying host CPUs.

Signed-off-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Tested-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- resolved conflict with "Adding helper function for WFI",
         nuked stray hard tab ]