net: allow c/r of empty bridges in the container

Implementing c/r of bridges with slaves shouldn't be too hard (viz. the
comment), but this is all I need to for right now.

v2: remove extra debug statement
v3: * remember to close fd in dump_bridge
    * use "known" buffer length and snprintf for spath in dump_bridge
    * change brace style

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>