Device discovery fx subcommands

CommandDescription
get-deviceprint the current selected device name
get-device-addrprint the Fuchsia address of the current selected device
list-deviceslist all nearby devices (excluding those on zedboot)
netaddrget the address of a running fuchsia system
netlslist running fuchsia systems on the local network
set-deviceset the default device to interact with
unset-deviceunset the default device to interact with