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: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
