PCIE: fix regression with coldplugged multifunction device

PCIE is causing asserts each time a multifunction device is added
on command line (coldplug).

This is caused by
commit a66e657e18cd9b70e9f57ae5512c07faf2bc508f
    pci/pcie: convert PCIE hotplug to use hotplug-handler API
QEMU abort is caused by misplaced assertion, which should
be checked only when device is hotplugged.

Reference to regression report:
 http://www.mail-archive.com/qemu-devel@nongnu.org/msg216226.html

Fixes: a66e657e18cd9b70e9f57ae5512c07faf2bc508f

Reported-By: Nigel Kukard <nkukard+qemu@lbsd.net>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
