ppc/kvm: Tell the user what might be wrong when using bad CPU types with kvm-hv

Using a CPU type that does not match the host is not possible when using
the kvm-hv kernel module - the PVR is checked in the kernel function
kvm_arch_vcpu_ioctl_set_sregs_hv() and rejected with -EINVAL if it
does not match the host.
However, when the user tries to specify a non-matching CPU type, QEMU
currently only reports "kvm_init_vcpu failed: Invalid argument", and
this is of course not very helpful for the user to solve the problem.
So this patch adds a more descriptive error message that tells the
user to specify "-cpu host" instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
[Removed melodramatic '!' :)]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
