Fix capture-instance-metadata.sh

bios_vendor file now says Xen, no amazon string
to match on any longer.

cat /sys/devices/virtual/dmi/id/bios_vendor
Xen

however result.json has the info we need.

/run/cloud-init/result.json
{
 "v1": {
  "datasource": "DataSourceEc2Local",
  "errors": []
 }
}

Issue-Id: RELENG-3163

Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>
Change-Id: Iece691579622608616a6abc8d27b9ca7653ad9ae