Return packet statistics in virt_wifi dump_station

Instead of returning constants for "packets transmitted" and "packets
failed to transmit" keep track of the actual statistics on it.

This also only returns data from dump_station when it is actually
connected to the network.

Bug: 116027983
Change-Id: Ife3f35dae22ed19b93d9a7914484f04919af2b85
Signed-off-by: A. Cody Schuffelen <schuffelen@google.com>
Acked-by: Greg Hartman <ghartman@google.com>