KVM: ARM: psci fixes: handle signals and pause

If a CPU running a VCPU receives a signal the run loop in the kernel
would spin if an ON-message was never sent through psci.  This will
eventually have to be done through vcpu->requests, but we will deal with
this later.

Also fix comment on kvm_vcpu_arch structure.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>