KVM: PPC: Book3S HV P9: Add kvmppc_stop_thread to match kvmppc_start_thread

Small cleanup makes it a bit easier to match up entry and exit
operations.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-22-npiggin@gmail.com