virNetDevBridgeGet: Don't require users to virNetDevSetupControl

virNetDevBridgeGet: Don't require users to virNetDevSetupControl

So far, this function has just three callers. Two of them call
virNetDevSetupControl to create a socket that we can then
optionally use for ioctl() to fetch data. However, querying sysfs
is preferred. Therefore it doesn't make much sense to require
users to set up the socket if they don't even know it will be
used in favour of sysfs. We can set up the socket iff we need to.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>