xen/balloon: Support xend-based toolstack

xen/balloon: Support xend-based toolstack

commit 3aa6c19d2f38be9c6e9a8ad5fa8e3c9d29ee3c35 upstream.

Xend-based toolstacks don't have static-max entry in xenstore. The
equivalent node for those toolstacks is memory_static_max.

Fixes: 5266b8e4445c (xen: fix booting ballooned down hvm guest)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: <stable@vger.kernel.org> # 4.13
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>