Drivers: hv: Execute shutdown in a thread context

Execute the shutdown code in a thread context. With recent changes made to the
shutdown code, shutdown code cannot be invoked from an interrupt context.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>