runqemu: Add support for kvm on aarch64

The main issue is to make the x86 checks apply to x86 targets only. We may
end up with better checks on other architectures but this adapts the code to
allow for that and its still controlled by whether QB_CPU_KVM is set.

The code needed minor refactoring so the qemu-system-XXX name is set
earlier so the kvm code can use it.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>