qemu: command: Replace caps check for QEMU_CAPS_MACH_VIRT_GIC_VERSION by arch check

qemu: command: Replace caps check for QEMU_CAPS_MACH_VIRT_GIC_VERSION by arch check

QEMU_CAPS_MACH_VIRT_GIC_VERSION is always asserted for VIR_ARCH_AARCH64.

Note that this patch is a direct conversion of the logic originally
residing in the capabilities code. A better coversion would be (based on
whether it is available for just AARCH64 or also ARM) to base it on the
guest architecture.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>