Add suspend() and resume() functions so the Hyper-V virtual keyboard
can participate in VM hibernation.

Note that the keyboard is a "wakeup" device that could abort an in-progress
hibernation if there is keyboard event.  No attempt is made to suppress this
behavior.  If desired, a sysadmin can disable the keyboard as a wakeup device
using standard mechanisms such as:

echo disabled > /sys/bus/vmbus/drivers/hyperv_keyboard/XXX/power/wakeup
(where XXX is the device's GUID)
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>