Use strings when reporting VM's memory stats

This patch changes the API to use strings when reporting memory and
balloon stats for a VM. This fixes an issue with xml-rpc that
can only transfer 32 bit signed numbers.

The maximum memory value when using KiB is about 2TB and we
already hit that.

Change-Id: I60b52fface17e5023c666ebed24dd5fc6b204dbc
Resolves: rhbz#974917
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17086
Reviewed-by: Dan Kenigsberg <danken@redhat.com>