Fix typo in code and comments

Replace writeable -> writable

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
diff --git a/block.c b/block.c
index f731c7a..8767d31 100644
--- a/block.c
+++ b/block.c
@@ -455,7 +455,7 @@
     open_flags = flags & ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
 
     /*
-     * Snapshots should be writeable.
+     * Snapshots should be writable.
      */
     if (bs->is_temporary) {
         open_flags |= BDRV_O_RDWR;