virt: sampling: make sure balloon values are str

This patch avoids XML-RPC integer overflows by ensuring
all the balloon stats are string values.

Change-Id: Ia71ceab52c1247a94ab3a61b3bbafec76106d789
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28954
Reviewed-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>