qemuimg: create size should be a string

execCmd requires the command list items to be strings.

Change-Id: I2173165363f3f75485226671eeef7b2a3a3cc1e2
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26868
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>