powerpc: Fix up RTAS invocation for new qemu versions

In order to call RTAS functions on powerpc kvm-unit-tests relies on the
RTAS blob supplied by qemu.  But new versions of qemu don't supply an RTAS
blob: since the normal way for guests to get RTAS is to call the guest
firmware's instantiate-rtas function, we now rely on that guest firmware
to provide the RTAS code itself.

But qemu-kvm-tests bypasses the usual guest firmware to just run itself,
so we can't get the rtas blob from SLOF.

But.. in fact the RTAS blob under qemu is a bit of a sham anyway - it's
a tiny wrapper that forwards the RTAS call to a hypercall.  So, we can
just invoke that hypercall directly.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20191004103844.32590-1-david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>