xen: use static initializers in xen-balloon.c

There is no need to use dynamic initializaion, it just confuses the reader.
Switch to static initializers like its used in other files.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
[v2: Rebased on v3.0]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>