virtual-device: Add non-image mock devices

There are two variants one with storage and identify support and the
other without storage.

It implements the following commands:
 * INSERT id
 * REMOVE id
 * SCAN id
 * ERROR error-code
 * LIST (returns saved print)

The INSERT/REMOVE/LIST commands are only available in the storage
driver. The SCAN command emulates presenting a finger.

These commands can be send ahead of time, and will be queued and
processed when appropriate. i.e. for INSERT/REMOVE that is immediately
when possible, for SCAN/ERROR processing is delayed.

The LIST command is always processed immediately.

Note that only a single command can be send per socket connection and
the command must be send in a single message. The socket will be closed
after the command has been processed.

Co-authored-by: Bastien Nocera <hadess@hadess.net>
Co-authored-by: Marco Trevisan (Treviño) <mail@3v1n0.net>

tests/meson.build

@@ -13,7 +13,11 @@ envs.prepend('LD_LIBRARY_PATH', join_paths(meson.build_root(), 'libfprint'))
 envs.set('FP_DEVICE_EMULATION', '1')
 
 # Set a colon-separated list of native drivers we enable in tests
-envs.set('FP_DRIVERS_WHITELIST', 'virtual_image')
+envs.set('FP_DRIVERS_WHITELIST', ':'.join([
+    'virtual_image',
+    'virtual_device',
+    'virtual_device_storage',
+]))
 
 envs.set('NO_AT_BRIDGE', '1')