kvm__emulate_io: Don't fall through from IO in to IO out if no handler

If an IO port device has no io_in handler, kvm__emulate_io would fall
through and call the io_out handler instead.  Fix to only call the
handler for the appropriate direction.

If no handler exists, kvm__emulate_io will automatically treat it as an
IO error (due to the default "ret = false").

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>