backend: sysprep custom variables should be in CDATA section

for handling special characters we need to put all inputs inside xml
CDATA section.

Change-Id: I941e1422077383386ee788e75ae5ad38d1de39a2
Bug-Url: https://bugzilla.redhat.com/1122160
Signed-off-by: Shahar Havivi <shaharh@redhat.com>