powerpc/e500/qemu-e500: allow core to idle without waiting

This means an idle guest won't needlessly consume an entire core on
the host, waiting for work to show up.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Acked-by: Scott Wood <oss@buserror.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220112112459.1033754-1-troglobit@gmail.com