vvfat: Use cache=unsafe

The qcow file used for write support in vvfat is a temporary file,
so we can use cache=unsafe there. Without this, write support is just
too slow to be of any use.

Signed-off-by: Kevin Wolf <mail@kevin-wolf.de>
(cherry picked from commit 35ccd8aed64727dbefa1b274a8000b46318bfea1)