The command line I am using is:
sudo virt-install --name=fc15guest --ram=1024 --os-type=linux \ --os-variant=fedora12 \ --location=/distrocds/Fedora/Fedora-15-x86_64-DVD.iso \ --disk="path=/dev/sauron/fc15guest"
I have never used virt-install or Fedora 15 but, maybe you have to do the same here like when installing RHEL6 using the iso image on a NFS server: In the directory where the ISO is, you have to create a directory called images and put the install.img from the DVD there. For RHEL6.1 the dirlisting looks like this:
[root@blubb i386]# ll -R .: total 3496772 drwxr-xr-x 2 root root 4096 May 19 18:20 images -rw-r--r-- 1 root root 204 May 19 18:13 MD5SUMS -rw-r--r-- 1 root root 218103808 May 12 12:53 rhel-server-6.1-i386-boot.iso -rw-r--r-- 1 root root 3066486784 May 13 07:13 rhel-server-6.1-i386-dvd.iso -rw-r--r-- 1 root root 292583424 May 13 15:33 rhel-server-supplementary-6.1-i386-dvd.iso
./images: total 132152 -rw-r--r-- 1 root root 135184384 May 11 00:30 install.img
Just a guess but you could give it a try.
Simon