Device discovery fx subcommands

Command	Description
get-device	print the current selected device name
get-device-addr	print the Fuchsia address of the current selected device
list-devices	list all nearby devices (excluding those on zedboot)
netaddr	get the address of a running fuchsia system
netls	list running fuchsia systems on the local network
set-device	set the default device to interact with
unset-device	unset the default device to interact with