VMware balloon: Treat init like reset

Unify the behavior of the first start of the balloon and a reset. Also on
unload, declare that the balloon driver does not have any capabilities
anymore.

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