Don't expose 'vnet%d' to the user

Don't expose 'vnet%d' to the user

https://bugzilla.redhat.com/517371

Matt Booth points out that if you use a non-existent bridge name when
start a guest you get a weird error message:

  Failed to add tap interface 'vnet%d' to bridge 'virbr0'

and dev='vnet%d' appears in the dumpxml output.

Fix that by not including 'vnet%d' in the error message and freeing the
'vnet%d' string if adding the tap device to the bridge fails.

* src/qemu_conf.c, src/uml_conf.c: fix qemudNetworkIfaceConnect()
  and umlConnectTapDevice() to not expose 'vnet%d' to the user