virtual-device: Add non-imaging mock driver

Has 2 variants, one that supports identify, one that doesn't.

The mode of operation for this device is:
- Run fprintd with "FP_VIRTUAL_DEVICE=/tmp/fprint.socket"
- As root:
FP_VIRTUAL_DEVICE=/tmp/fprint.socket ./sendvirtcmd.py "ADD left-middle testuser success"
FP_VIRTUAL_DEVICE=/tmp/fprint.socket ./sendvirtcmd.py "ADD right-middle testuser failure"
- As testuser:
fprintd-enroll -f left-middle-finger
fprintd-enroll -f right-middle-finger

Now, across fprintd restarts (as it has its own storage), verification
will always succeed for user "testuser" with the "left-middle-finger".

Using the FP_VIRTUAL_DEVICE_IDENT envvar instead will allow
identification, with the first found "successful" finger being presented
as the identified one.

meson.build

@@ -88,7 +88,7 @@ cairo_dep = dependency('cairo', required: false)
 
 # Drivers
 drivers = get_option('drivers').split(',')
-virtual_drivers = [ 'virtual_image' ]
+virtual_drivers = [ 'virtual_image', 'virtual_device' ]
 default_drivers = [
     'upektc_img',
     'vfs5011',