Rather than having widget/widget2/widget_callback fields in the struct
alsa_elem, have a list of callbacks and allow private data to be
passed to callbacks.
Rather than retrieving the firmware version in alsa_scan_cards(), use
get_elem_by_name() to check if the control is present.
Update alsa-sim.c alsa_config_to_new_elem() to read iface=CARD
controls.
The level meters don't work correctly on earlier kernel versions, so
they shouldn't be displayed in that case. The "Firmware Version" ALSA
control was added to the kernel at the same time as the level meters
were fixed, so use the presence of that to determine whether or not to
enable the menu item.
