Suggested by Paolo & Greg, add 'events' device attribute that can be
used to limit which capabilities the driver uses.

Finally, the pvpanic guest driver works by the limitation of both
device capability and user setting.
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20210110115358.79100-3-pizhenwei@bytedance.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>