[patch] spec: Update alsa namespace to include virtual device

Hi

Thanks to the great work of Bluez-guys, bluez-utils audio service is
now able to deal with several audio devices. Those Bluetooth devices
are accessed by a "bluetooth" ALSA plugin (see
http://wiki.bluez.org/wiki/HOWTO/AudioDevices)

There is only a few missing pieces to make those devices discoverable with HAL.

The following patch changes the "alsa" namespace so that "virtual
devices" can also be described (a former approach was to use
alsa.virtual or alsa_virtual new namespace, but alsa.virtual does not
inherit alsa property, and both namespaces have a lot in common).

Thus, a  mandatory property "is_hardware" is introduced. If this value
is true, the current mandatory keys remain mandatory. If not, then the
client should use "device_name" to "open" the device.

Arguable:
- should "device_name" be optional? (currently, application hard-code
the prefix "hw")
- "service" is used to make a difference between a2dp and hfp/hsp
- "device_name", and "service" key naming

Cheers,
-- 
Marc-André Lureau