add format= to drive options (CVE-2008-2004)

It is possible for a guest with a raw formatted disk image to write a
header to that disk image describing another format (such as qcow2).
Stopping and subsequent restart of the guest will cause qemu to detect
that format, and could allow the guest to read any host file if qemu is
sufficiently privileged (typical in virt environments).

The patch defaults to existing behaviour (probing based on file contents),
so it still requires the mgmt app (e.g. libvirt xml) to pass a new
"format=raw" parameter for raw disk images.

Originally noted by Avi Kivity, patch from Chris Wright.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4277 c046a42c-6fe2-441c-8c8c-71466251a162
