Xen: fix whitespaces,tabs coding style issue in drivers/xen/balloon.c

This is a patch to the balloon.c file that fixed up
whitespaces, tabs errors found by the checkpatch.pl tools.

Signed-off-by: Ruslan Pisarev <ruslan@rpisarev.org.ua>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>