KVM: ARM: Remove unused __kvm_va_to_pa function

After the MMIO decoding code has been removed, this function is also no
longer used, so let's get rid of it.

Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>
KVM: ARM: Document kvm_call_hyp and fix __kvm_vcpu_run return value

The kvm_call_hyp calling convention and semantics should be described.

Also, just in case someone looks at the return value from __kvm_vcpu_run
without casting it to an unsigned long, we clear the upper bits.

Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>