qemu-img: use the heap instead of the huge stack array for win32

The default stack size of PE is 1MB on win32 and IO_BUF_SIZE in
img_convert() & img_rebase() is 2MB, so qemu-img will crash when doing
"convert" & "rebase" on win32.
Although we can improve the stack size of PE to resolve it, I think we
should avoid using the huge stack variables.

Signed-off-by: TeLeMan <geleman@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>