We have a host in chaos lab with 3 devices attached to it.
root@chromeos3-row1-rack2-host1:~# adb devices
List of devices attached 
ZX1G22J89F	device
ZX1G22SHXX	device
002ee5170e6ca34e	device

Am trying to run autotests with the -s options as follows:

./site_utils/test_droid.py -r chromeos3-row1-rack2-host1.cros -s ZX1G22J89F network_WiFi_ChaosConnectDisconnect.open

romeos3-row1-rack2-host1.cros'
14:34:46 INFO | autoserv| [stderr] error: more than one device and emulator
14:34:46 INFO | autoserv| Unable to reset the device adb daemon connection: command execution error
14:34:46 INFO | autoserv| * Command:
14:34:46 INFO | autoserv| /usr/bin/ssh -a -x  -o ControlPath=/tmp/_autotmp_ST9WSHssh-master/socket
14:34:46 INFO | autoserv| -o StrictHostKeyChecking=no -o UserKnownHostsFile=/tmp/tmpVErWgk -o
14:34:46 INFO | autoserv| BatchMode=yes -o ConnectTimeout=30 -o ServerAliveInterval=300 -l root -p
14:34:46 INFO | autoserv| 22 chromeos3-row1-rack2-host1.cros " adb root"
14:34:46 INFO | autoserv| Exit status: 1
14:34:46 INFO | autoserv| Duration: 0.0189399719238
14:34:46 INFO | autoserv| 
14:34:46 INFO | autoserv| stderr:
14:34:46 INFO | autoserv| error: more than one device and emulator.
14:34:46 INFO | autoserv| Unlocking hosts: set(['chromeos3-row1-rack1-host12', 'chromeos3-chaosvm9'])
14:34:46 INFO | autoserv| host_modifier: updated_hosts = set(['chromeos3-row1-rack1-host12', 'chromeos3-chaosvm9'])
14:34:48 INFO | autoserv| Exception escaped control file, job aborting:
14:34:48 INFO | autoserv| Traceback (most recent call last):
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/server/server_job.py", line 684, in run
14:34:48 INFO | autoserv| self._execute_code(server_control_file, namespace)
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/server/server_job.py", line 1182, in _execute_code
14:34:48 INFO | autoserv| execfile(code_file, namespace, namespace)
14:34:48 INFO | autoserv| File "/tmp/test_that_results_03ypaa/results-1-network_WiFi_ChaosConnectDisconnect.open/control.srv", line 32, in <module>
14:34:48 INFO | autoserv| parallel_simple(run_chaos_open, machines)
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/server/subcommand.py", line 93, in parallel_simple
14:34:48 INFO | autoserv| function(arg)
14:34:48 INFO | autoserv| File "/tmp/test_that_results_03ypaa/results-1-network_WiFi_ChaosConnectDisconnect.open/control.srv", line 29, in run_chaos_open
14:34:48 INFO | autoserv| runner.run(job)
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/server/cros/chaos_lib/chaos_runner.py", line 128, in run
14:34:48 INFO | autoserv| False)) as client:
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/server/cros/network/wifi_client.py", line 332, in __init__
14:34:48 INFO | autoserv| inherit_interfaces=True)
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/server/site_linux_system.py", line 71, in __init__
14:34:48 INFO | autoserv| self.__setup()
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/server/site_linux_system.py", line 82, in __setup
14:34:48 INFO | autoserv| cmd_iw = path_utils.must_be_installed('/usr/sbin/iw', host=self.host)
14:34:48 INFO | autoserv| File "/usr/local/ssd/chromiumos/src/third_party/autotest/files/client/common_lib/cros/path_utils.py", line 65, in must_be_installed
14:34:48 INFO | autoserv| raise error.TestError(error_msg)
14:34:48 INFO | autoserv| TestError: Unable to find /usr/sbin/iw on chromeos3-row1-rack2-host1.cros