network: eliminate unnecessary labels

network: eliminate unnecessary labels

All these cleanup/error labels were reduced to having just "return
ret" by a previous patch, so get rid of them and return directly.

This patch coincidentally fixes a bug in
networkFindUnusedBridgeName(), where we would log an error yet still
return success if we failed to find a single unused "virbrNNN" name
after checking all values of "N" from 0 - 256. Said bug was introduced
when that function was originally written, in commit a28d3e485f
(libvirt 1.2.15, 2015)

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>