[contrib] Allow "cow" script to create the SAN device itself

Allow for an invocation pattern such as:

  ./cow /path/to/image/file vblade 1 2 eth0

Signed-off-by: Michael Brown <mcb30@ipxe.org>

contrib/vm/cow

 
 trap cleanup EXIT
 
-imgfile=$1
-if [ -z "$imgfile" ] ; then
-    echo Syntax: $0 /path/to/image/file
+imgfile=$1 ; shift
+command=$1 ; shift
+if [ -z "$imgfile" -o -z "$command" ] ; then
+    echo Syntax: $0 /path/to/image/file command [args..]
     exit 1
 fi
 
 
 # Wait until killed
 echo "Created $cowlink"
-while : ; do sleep 2147483647 ; done
+$command "$@" $cowlink