ppc/pnv.c: add a friendly warning when accel=kvm is used

ppc/pnv.c: add a friendly warning when accel=kvm is used

If one tries to use -machine powernv9,accel=kvm in a Power9 host, a
cryptic error will be shown:

qemu-system-ppc64: Register sync failed... If you're using kvm-hv.ko, only "-cpu host" is possible
qemu-system-ppc64: kvm_init_vcpu: kvm_arch_init_vcpu failed (0): Invalid argument

Appending '-cpu host' will throw another error:

qemu-system-ppc64: invalid chip model 'host' for powernv9 machine

The root cause is that in IBM PowerPC we have different specs for the bare-metal
and the guests. The bare-metal follows OPAL, the guests follow PAPR. The kernel
KVM modules presented in the ppc kernels implements PAPR. This means that we
can't use KVM accel when using the powernv machine, which is the emulation of
the bare-metal host.

All that said, let's give a more informative error in this case.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20211130133153.444601-2-danielhb413@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>