qemu_img_format : use raw for as default for other storage (nexenta, sheepdog, ...)

qemu_img_format : use raw for as default for other storage (nexenta, sheepdog, ...)

Seem that something have change in qemu.

iscsi qemu block driver need to use raw, or we get a segfault. (like rbd).

sheepdog works also with .raw, so we don't need anymore specific storage format, we can use raw.

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>