mshv_eventfd: convert to CLASS(fd)

similar to 66635b07 ("assorted variants of irqfd setup: convert
to CLASS(fd)") a year ago...

Acked-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>