Issue warning when deprecated drive parameter boot=on|off is used

Releases of qemu-kvm will be interrupted at qemu 1.3.0.
Users should switch to plain qemu releases.
To avoid breaking scenarios which are setup with command line
options specific to qemu-kvm, port these switches from qemu-kvm
to qemu.git.

Port drive boot option. From the qemu-kvm original commit message:

We do not want to maintain this option forever. It will be removed after
a grace period of a few releases. So warn the user that this option has
no effect and will become invalid soon.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
diff --git a/qemu-config.c b/qemu-config.c
index e854fff..3154cac 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -114,6 +114,10 @@
             .name = "copy-on-read",
             .type = QEMU_OPT_BOOL,
             .help = "copy read data from backing file into image file",
+        },{
+            .name = "boot",
+            .type = QEMU_OPT_BOOL,
+            .help = "(deprecated, ignored)",
         },
         { /* end of list */ }
     },