Bluetooth: Add get_connections managment interface command

This patch adds a get_connections command to the management interface.
With this command userspace can get the current list of connected
devices. Typically this command would only be used once when enumerating
existing adapters. After that the connected and disconnected events are
used to track connections.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>