pci: Reduce scope of error injection

No one outside of pcie_aer.h was using error injection; mark them
static for internal use.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20170427215821.19397-3-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
