Realism in virt_wifi driver methods.

 - Only return a probe response, not a beacon for the fake wifi network.
 - Remove the delay between the probe response and the scan result.
 - Only check the wifi network bss after the artificial delay.
 - Only return station info for the fake wifi network bss.

Change-Id: Iefcaa4e886ba77a3a57849d5c3234a2706a40a0e
Bug: 116027983
Signed-off-by: A. Cody Schuffelen <schuffelen@google.com>
Acked-by: Greg Hartman <ghartman@google.com>
Acked-by: Alistair Strachan <astrachan@google.com>