misc: pvpanic: introduce device capability

According to pvpanic spec:
https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/specs/pvpanic.txt



The guest should determine pvpanic capability by RDPT, so initialize
capability during device probing. There is no need to register panic
notifier callback function if no events supported.

Before sending event to host side, check capability firstly.

Suggested by Greg KH, use sysfs to expose capability to user space,
also add new sysfs attribute in document.

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20210110115358.79100-2-pizhenwei@bytedance.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>