KVM: PPC: mmio: Return to guest after emulation failure

If MMIO emulation fails we don't want to crash the whole guest by
returning to userspace.

The original commit bbf45ba5 ("KVM: ppc: PowerPC 440 KVM
implementation") added a todo:

  /* XXX Deliver Program interrupt to guest. */

and later the commit d69614a2 ("KVM: PPC: Separate loadstore
emulation from priv emulation") added the Program interrupt injection
but in another file, so I'm assuming it was missed that this block
needed to be altered.

Also change the message to a ratelimited one since we're letting the
guest run and it could flood the host logs.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220125215655.1026224-5-farosas@linux.ibm.com