ppc/vhyp: reset exception state when handling vhyp hcall

Convention is to reset the exception_index and error_code after handling
an interrupt. The vhyp hcall handler fails to do this. This does not
appear to have ill effects because cpu_handle_exception() clears
exception_index later, but it is fragile and inconsistent. Reset the
exception state after handling vhyp hcall like other handlers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>