block: Make qiov match the request size until EOF

If a read request goes across EOF, the block driver sees a shortened
request that stops at EOF (the rest is memsetted in block.c), however
the original qiov was used for this request.

This patch makes the qiov size match the request size, avoiding a
potential buffer overflow in raw-posix.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>