qemu-iotests: add poke_file utility function

The new poke_file function sets bytes at an offset in a file given a
printf-style format string.  It can be used to corrupt an image file for
test coverage of error paths.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/common.rc

@@ -34,6 +34,12 @@ dd()
    fi
 }
 
+# poke_file 'test.img' 512 '\xff\xfe'
+poke_file()
+{
+    printf "$3" | dd "of=$1" bs=1 "seek=$2" conv=notrunc &>/dev/null
+}
+
 # we need common.config
 if [ "$iam" != "check" ]
 then