ui/gtk: fix "Copy" menu item segfault

The "Copy" menu item copies VTE terminal text to the clipboard.  This
only works with VTE terminals, not with graphics consoles.

Disable the menu item when the current notebook page isn't a VTE
terminal.

This patch fixes a segfault.  Reproducer: Start QEMU and click the Copy
menu item when the guest display is visible.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20161214142518.10504-1-stefanha@redhat.com
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>