vhost-scsi: Improve error reporting for invalid vhostfd

We get two error messages: one from monitor_handle_fd_param2(), and
another one from vhost_scsi_realize().  The second one gets suppressed
in QMP context.

That's because monitor_handle_fd_param() calls qerror_report_err().
Calling qerror_report_err() is always inappropriate in realize
methods, because it doesn't return the Error object.  It either
reports the error to stderr or the human monitor, or it stores it in
the QMP monitor, where it makes the QMP command fail even when the
realize method ignores the error and succeeds.  Fortunately,
vhost_scsi_realize() doesn't do that.

Fix by switching to monitor_handle_fd_param2().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
