tests/qemu-iotests: add QEMU_IOTESTS_REGEN=1 to update reference file

tests/qemu-iotests: add QEMU_IOTESTS_REGEN=1 to update reference file

When developing an I/O test it is typical to add some logic to the
test script, run it to view the output diff, and then apply the
output diff to the reference file. This can be drastically simplified
by letting the test runner update the reference file in place.

By setting 'QEMU_IOTESTS_REGEN=1', the test runner will report the
failure and show the diff, but at the same time update the reference
file. So next time the I/O test is run it will succeed.

Continuing to display the diff when updating the reference gives the
developer a chance to review what was changed.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220304193610.3293146-7-berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>