hw/scsi: Use error_setg_file_open() for a better error message

hw/scsi: Use error_setg_file_open() for a better error message

The error message changes from

    vhost-scsi: open vhost char device failed: REASON

to

    Could not open '/dev/vhost-scsi': REASON

I think the exact file name is more useful to know than the file's
purpose.

We could put back the "vhost-scsi: " prefix with error_prepend().  Not
worth the bother.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20251121121438.1249498-7-armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>