I'm getting:

    could not configure /dev/net/tun (tap%d): Operation not permitted

When the ioctl() fails, ifr.ifr_name will most likely not be overwritten.
So we better only use it when ifname contains a string.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>