hw/xen/xen_pvdev: Include qemu/log.h for qemu_log_vprintf()

Olaf Hering reported a build failure due to an undefined reference
to 'qemu_log_vprintf'. Explicitely including qemu/log.h seems to
fix the issue.
Signed-off-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NStefano Stabellini <sstabellini@kernel.org>
Acked-by: NStefano Stabellini <sstabellini@kernel.org>
Tested-by: NOlaf Hering <olaf@aepfle.de>

hw/xen/xen_pvdev.c

@@ -18,7 +18,7 @@
  */
 
 #include "qemu/osdep.h"
-
+#include "qemu/log.h"
 #include "hw/xen/xen_backend.h"
 #include "hw/xen/xen_pvdev.h"