multi-process: Fix pci_proxy_dev_realize() error handling

multi-process: Fix pci_proxy_dev_realize() error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

pci_proxy_dev_realize() is wrong that way: it passes @errp to
qio_channel_new_fd() without checking for failure.  If it runs into
another failure, it trips error_setv()'s assertion.

Fix it to check for failure properly.

Fixes: 9f8112073aad8e485ac012ee18809457ab7f23a6
Cc: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Cc: Jagannathan Raman <jag.raman@oracle.com>
Cc: John G Johnson <john.g.johnson@oracle.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-5-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Jagannathan Raman <jag.raman@oracle.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>