virtio-blk: fix minimum number of S/G elements

We need at least one S/G element to operate properly, as does the block
layer which increments it to one anyway.  We hit this due to a qemu
bug which advertises a sg_elements of 0 under some circumstances.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (tweaked logic)