block/curl: Improve type safety of s->timeout.

qemu_opt_get_number returns a uint64_t, and curl_easy_setopt expects a
long (not an int).  There is no warning about the latter type error
because curl_easy_setopt uses a varargs argument.

Store the timeout (which is a positive number of seconds) as a
uint64_t.  Check that the number given by the user is reasonable.
Zero is permissible (meaning no timeout is enforced by cURL).

Cast it to long before calling curl_easy_setopt to fix the type error.

Example error message after this change has been applied:

$ ./qemu-img create -f qcow2 /tmp/test.qcow2 \
    -b 'json: { "file.driver":"https",
                "file.url":"https://foo/bar",
                "file.timeout":-1 }'
qemu-img: /tmp/test.qcow2: Could not open 'json: { "file.driver":"https", "file.url":"https://foo/bar", "file.timeout":-1 }': timeout parameter is too large or negative: Invalid argument

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
