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: I3a834c265f513fd7c9628cf0c7179f8118904447
Bug-Url: https://bugzilla.redhat.com/1132191
Signed-off-by: Shahar Havivi <shaharh@redhat.com>