usb-host: scan for usb devices when the vm starts

Commit a844ed84 leads to usb-host
detecting devices not right after qemu startup because the guest
isn't running yet.  Instead they are found on the first of the
regular usb device poll runs.  Which is too late for seabios to see
them, so booting from usb sticks fails.

Fix this by adding a vm state change handler which triggers a device
scan when the vm is started.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/host-linux.c

@@ -1738,6 +1738,7 @@ static int usb_host_scan(void *opaque, USBScanFunc *func)
 }
 
 static QEMUTimer *usb_auto_timer;
+static VMChangeStateEntry *usb_vmstate;
 
 static int usb_host_auto_scan(void *opaque, int bus_num,
                               int addr, const char *port,
@@ -1792,6 +1793,13 @@ static int usb_host_auto_scan(void *opaque, int bus_num,
     return 0;
 }
 
+static void usb_host_vm_state(void *unused, int running, RunState state)
+{
+    if (running) {
+        usb_host_auto_check(unused);
+    }
+}
+
 static void usb_host_auto_check(void *unused)
 {
     struct USBHostDevice *s;
@@ -1820,6 +1828,9 @@ static void usb_host_auto_check(void *unused)
         }
     }
 
+    if (!usb_vmstate) {
+        usb_vmstate = qemu_add_vm_change_state_handler(usb_host_vm_state, NULL);
+    }
     if (!usb_auto_timer) {
         usb_auto_timer = qemu_new_timer_ms(rt_clock, usb_host_auto_check, NULL);
         if (!usb_auto_timer) {