When I tried to use QCow image, I found that only each second boot is
successful. As I discovered, this is caused by wrong handling old qcow
tapdisk images. Extended header flag is not stored correctly so the
blktap tries to change endian fo L1 table on each startup.

From: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>