Introduce machine property "audiodev"

Many machine types have default audio devices with no way to set the underlying
audiodev.  Instead of adding an option for each and every one of them, this new
property can be used as a default during machine initialisation when creating
such devices.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
[Make the property optional, instead of including it in all machines. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>