qdev: Display warning about unused -global

This can help a user understand why -global was ignored.

For example: with "-vga cirrus"; "-global vga.vgamem_mb=16" is just
ignored when "-global cirrus-vga.vgamem_mb=16" is not.

This is currently clear when the wrong property is provided:

out/x86_64-softmmu/qemu-system-x86_64 -global cirrus-vga.vram_size_mb=16 -monitor pty -vga cirrus
char device redirected to /dev/pts/20 (label compat_monitor0)
qemu-system-x86_64: Property '.vram_size_mb' not found
Aborted (core dumped)

vs

out/x86_64-softmmu/qemu-system-x86_64 -global vga.vram_size_mb=16 -monitor pty -vga cirrus
char device redirected to /dev/pts/20 (label compat_monitor0)
VNC server running on `::1:5900'
^Cqemu: terminating on signal 2

Signed-off-by: Don Slutz <dslutz@verizon.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/vl.c b/vl.c
index 709d8cd..bda88f7 100644
--- a/vl.c
+++ b/vl.c
@@ -4532,6 +4532,8 @@
         }
     }
 
+    qdev_prop_check_global();
+
     if (incoming) {
         Error *local_err = NULL;
         qemu_start_incoming_migration(incoming, &local_err);