imageSharing: return proper size in httpGetSize

When importing images from OpenStack Glance the Content-Length is 0
so we need to override the value with the content of the custom header
X-Image-Meta-Size.

Change-Id: Icbcc601767d7f5b044b9e0e32f35abccdfb5746b
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19222
Reviewed-by: Dan Kenigsberg <danken@redhat.com>