Make sure the balloon info uses numbers when provided by VDSM

VDSM needs to strings for transporting large numeric values over
xml-rpc. Make sure MoM can properly process those.

The GuestBalloon collector has to filter the fields as well
to only export the required numbers to the Monitor. VDSM added
balloon_target that we do not need here.

Change-Id: I74b227a5d945397a69702fc0758d864b33014fc6
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17207
Reviewed-by: Adam Litke <agl@us.ibm.com>