hw/arm/virt: Support using SMC for PSCI

hw/arm/virt: Support using SMC for PSCI

If we are giving the guest a CPU with EL2, it is likely to
want to use the HVC instruction itself, for instance for
providing PSCI to inner guest VMs. This makes using HVC
as the PSCI conduit for the outer QEMU a bad idea. We will
want to use SMC instead is this case: this makes sense
because QEMU's PSCI implementation is effectively an
emulation of functionality provided by EL3 firmware.

Add code to support selecting the PSCI conduit to use,
rather than hardcoding use of HVC.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 1483977924-14522-15-git-send-email-peter.maydell@linaro.org