Hotunplug for all other devices are warning the user when the hotunplug
is already in progress. Do the same for PCI devices in
spapr_pci_unplug_request().

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20210226163301.419727-5-danielhb413@gmail.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>