Drivers: hv: vmbus: Add /sys/bus/vmbus/hibernation

Drivers: hv: vmbus: Add /sys/bus/vmbus/hibernation

When a Linux VM runs on Hyper-V, if the host toolstack doesn't support
hibernation for the VM (this happens on old Hyper-V hosts like Windows
Server 2016, or new Hyper-V hosts if the admin or user doesn't declare
the hibernation intent for the VM), the VM is discouraged from trying
hibernation (because the host doesn't guarantee that the VM's virtual
hardware configuration will remain exactly the same across hibernation),
i.e. the VM should not try to set up the swap partition/file for
hibernation, etc.

x86 Hyper-V uses the presence of the virtual ACPI S4 state as the
indication of the host toolstack support for a VM. Currently there is
no easy and reliable way for the userspace to detect the presence of
the state (see https://lkml.org/lkml/2020/12/11/1097).  Add
/sys/bus/vmbus/hibernation for this purpose.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20210107014552.14234-1-decui@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>