Always treat floppy images as raw

Always treat floppy images as raw

The fix for the image format guessing vulnerability breaks raw floppy
images.  In our environment it is difficult to apply the same
configuration change as envisaged upstream, so instead we arrange to
always treat a floppy image as raw.

Two-line patch supplied by James Harper <james.harper@bendigoit.com.au>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit f2ce5c35f60bb606bd3ddb55c361bb8a9aad5c5c)

[ Also originally committed as 342846c1eecc972fcfae84d9d1ae65a318101792
  Mistakenly dropped from qemu-xen-unstable.git and now reapplied.
  Thanks to report from James Harper.   -iwj ]