qemu: hotplug: wait for the tray to eject only for drives with a tray

Use the detected tray presence flag to trigger the tray waiting code
only if the given storage device in qemu reports to have a tray.

This is necessary as the floppy device lost it's tray as of qemu commit:

commit abb3e55b5b718d6392441f56ba0729a62105ac56
Author: Max Reitz <mreitz@redhat.com>
Date:   Fri Jan 29 20:49:12 2016 +0100

    Revert "hw/block/fdc: Implement tray status"

(cherry picked from commit 72a7ff6b50)

src/qemu/qemu_hotplug.c

@@ -209,6 +209,7 @@ qemuDomainChangeEjectableMedia(virQEMUDriverPtr driver,
     int ret = -1, rc;
     char *driveAlias = NULL;
     qemuDomainObjPrivatePtr priv = vm->privateData;
+    qemuDomainDiskPrivatePtr diskPriv = QEMU_DOMAIN_DISK_PRIVATE(disk);
     const char *format = NULL;
     char *sourcestr = NULL;
 
@@ -237,8 +238,9 @@ qemuDomainChangeEjectableMedia(virQEMUDriverPtr driver,
     if (qemuDomainObjExitMonitor(driver, vm) < 0)
         goto cleanup;
 
-    /* If the tray change event is supported wait for it to open. */
-    if (virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_DEVICE_TRAY_MOVED)) {
+    /* If the tray is present and tray change event is supported wait for it to open. */
+    if (diskPriv->tray &&
+        virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_DEVICE_TRAY_MOVED)) {
         if (qemuHotplugWaitForTrayEject(driver, vm, disk, driveAlias, force) < 0)
             goto error;
     } else  {