VMware balloon: Show capabilities of balloon and resulting capabilities in the debug-fs node.

This helps with debugging vmw_balloon behavior, as it is clear what
functionality is enabled.

Acked-by: Andy King <acking@vmware.com>
Signed-off-by: Xavier Deguillard <xdeguillard@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>