qemu-iotests: add 036 autoclear feature bit test

This new test validates the autoclear feature bit behavior.  When QEMU
opens a qcow2v3 image file with an unknown autoclear feature bit the bit
should be cleared in the image file header.

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