vm: Modify memory units of numa node

This patch is based on http://gerrit.ovirt.org/#/c/25254/ to modify the
memory units. Since there is a limitation in xmlrpc to transfer large
integer, this patch modify the memory value to string format in MB unit.

Change-Id: I1d53406796c8080adba581b5588c6edcd5b4ce4c
Signed-off-by: Bruce Shi <xiao-lei.shi@hp.com>
Reviewed-on: http://gerrit.ovirt.org/27403
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>