Description of problem:
The tap_fd_to_name() method in xenner.c iterates over 65535 interface indexes
querying each one for its mac address. It compares against the expected mac
address for the TAP fd forwarded from libvirtd. The logic assumes that the TAP
device interface index will always be greater than the bridge device interface
index because the bridge device may have the same MAC too.

Unfortunately this assumption does not appear to hold true on my test system
where the bridge device is after the TAP device

24273 ioctl(5, SIOCGIFNAME, {ifr_index=27, ifr_name=???}) = -1 ENODEV (No such
device)
24273 ioctl(5, SIOCGIFNAME, {ifr_index=28, ifr_name="vnet0"}) = 0
24273 ioctl(5, SIOCGIFHWADDR, {ifr_name="vnet0", ifr_hwaddr=00:16:3e:58:eb:6f}) = 0
24273 ioctl(5, SIOCGIFNAME, {ifr_index=29, ifr_name=???}) = -1 ENODEV (No such
device)
24273 ioctl(5, SIOCGIFNAME, {ifr_index=30, ifr_name=???}) = -1 ENODEV (No such
device)
24273 ioctl(5, SIOCGIFNAME, {ifr_index=31, ifr_name=???}) = -1 ENODEV (No such
device)
24273 ioctl(5, SIOCGIFNAME, {ifr_index=32, ifr_name=???}) = -1 ENODEV (No such
device)
24273 ioctl(5, SIOCGIFNAME, {ifr_index=33, ifr_name="virbr0"}) = 0
24273 ioctl(5, SIOCGIFHWADDR, {ifr_name="virbr0", ifr_hwaddr=00:16:3e:58:eb:6f}) = 0
24273 ioctl(5, SIOCGIFNAME, {ifr_index=34, ifr_name=???}) = -1 ENODEV (No such
device)
24273 ioctl(5, SIOCGIFNAME, {ifr_index=35, ifr_name=???}) = -1 ENODEV (No such
device)


So, Xenner tries to then invoke a TUNSETPERSIST ioctl() on the bridge device &
fails.

Since we can't assume anything about interface ordering, Xenner needs a
different approach to distinguish a TAP device from a bridge device. One option
is to simply try TUNSETPERSIST on each interface with a matching MAC address. If
you get '-EINVAL' from the ioctl() then it indicates the interface is not a TAP
device & should be skipped.

Version-Release number of selected component (if applicable):
xenner-0.23 and xenner-0.25

How reproducible:
Sometimes

Steps to Reproduce:
1.
2.
3.
  
Actual results:


Expected results:


Additional info: