QemuOpts: add -set option

One use case will be file for drives (no filename quoting issues), i.e.

	-drive id=test,if=virtio
	-set drive.test.file=/vmdisk/test-virtio.img

It will work for any other option (assuming handled by QemuOpts) though.
Except for id= for obvious reasons ;).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-Id: 
diff --git a/qemu-config.h b/qemu-config.h
index 3ada418..7faec9b 100644
--- a/qemu-config.h
+++ b/qemu-config.h
@@ -1 +1,3 @@
 extern QemuOptsList qemu_drive_opts;
+
+int qemu_set_option(const char *str);