vhost: Clean up how VhostOpts method vhost_get_config() fails

vhost: Clean up how VhostOpts method vhost_get_config() fails

vhost_user_get_config() can fail without setting an error.  Unclean.
Its caller vhost_dev_get_config() compensates by substituting a
generic error then.  Goes back to commit 50de51387f "vhost:
Distinguish errors in vhost_dev_get_config()".

Clean up by moving the generic error from vhost_dev_get_config() to
all the failure paths that neglect to set an error.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-13-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[Sign of error_setg_errno()'s second argument fixed in both calls]