btproxy: Allow to select multiple BT controllers

btproxy: Allow to select multiple BT controllers

When running on a computer with a real Bluetooth controller (e.g. hci0) and
multiple emulators (e.g. hci1 and hci2) it isn't possible to use the
emulators with 2 test-runner vms.
If btproxy is started without index parameter the first test-runner will
use hci0, and btprox can't be started with multiple index parameters
(e.g. -i1 -i2).

This patch keeps the old beahvior when used without -i option, in this case
it will try to use the first controller available.
It also allows to select multiple controllers to be used by btproxy.

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>