feat(input): daemon sets up the host->guest input-linux bridge via QMP

The uinput devices the input adapter creates were never forwarded into the
guest: the input-linux bridge was external (manual monitor object_add), so it
was lost on every reconfigure and whenever the kernel-assigned device numbers
changed. Make the vmhost seam -- which already owns the VM's single QMP
connection -- add the input-linux objects itself on reaching READY (A=keyboard
+abs with grab_all, B=mouse) and object_del them on teardown. The input adapter
publishes the uinput evdev paths into a per-endpoint home; discovery attaches
input before vmhost so the paths are ready. No second QMP socket or connection.

Also move the mouse buttons (incl. middle) and the wheel onto device B, so B is
a complete relative mouse and A is keyboard+abs only.

Bump 0.3.8.

src/si/input/include/driver.h

@@ -23,6 +23,8 @@ struct vmctl {
     int              ui_fd_a;   /* uinput driver: device A; -1 for QMP      */
     int              ui_fd_b;   /* uinput driver: device B (BOTH); -1       */
     int              ptr_mode;  /* uinput driver: VMCTL_PTR_*; 0 for QMP    */
+    char             ui_evdev_a[64];  /* uinput driver: /dev/input/eventN of A ("" if none) */
+    char             ui_evdev_b[64];  /* uinput driver: /dev/input/eventN of B ("" if none) */
 
     /* Held-state receipt: key/btn down-bits as THIS handle last actuated them
      * (not guest truth). Written only after a successful send in