vl: Always pass dvd_path to the driver and handle errors

[wilkovatch]

Closes #1964.

Side note: loading images via URLs works on Linux, but not on Windows, where it fails with the following messages:

fd=1672 is not a socket, AIO implementation is missing
fd=1672 is not a socket, AIO implementation is missing
curl: Connection timed out after 5002 milliseconds
warning: Could not read image for determining its format: Input/output error

(EDIT: looks like an upstream issue, the same happens on QEMU)
(EDIT 2: It's caused by this and the next 3 commits, a partial revert (for the non-socket case only) solves the issue, but that's outside the scope of this PR)