curl: Operate on zero-length file

curl: Operate on zero-length file

Another attempt to fix the bug 1596870.

When creating new disk backed by remote file accessed via HTTPS and the
backing file has zero length, qemu-img terminates with uniformative
error message:

    qemu-img: disk.qcow2: CURL: Error opening file:

While it may not make much sense to operate on empty file, other block
backends (e.g. raw backend for regular files) seem to allow it. This
patch fixes it for the curl backend and improves the reported error.

Signed-off-by: Tomáš Golembiovský <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>