Allow brAddTap to create a tap device that is down

Allow brAddTap to create a tap device that is down

An upcoming patch has a use for a tap device to be created that
doesn't need to be actually put into the "up" state, and keeping it
"down" keeps the output of ifconfig from being unnecessarily cluttered
(ifconfig won't show down interfaces unless you add "-a").

bridge.[ch]: add "up" as an arg to brAddTap()
uml_conf.c, qemu_command.c: add "up" (set to "true") to brAddTap() call.