Fix unattended_install.url on Fedora guests

My commit 756a1ff9a08ff2b1a16202a32e667b56204accc5 broke
unattended install with URL setting. This commit fixes
it in a very simplistic way, since I was tired to dig
into qemu devices code to try out adding unit=0/unit=1
suggested by Paolo, and also, this change would require
more testing with other qemu branches, such as RHEL5/RHEL6
before we know it is safe.

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>