hw/pci/pcie: Move hot plug capability check to pre_plug callback

hw/pci/pcie: Move hot plug capability check to pre_plug callback

Check for hot plug capability earlier to avoid removing devices attached
during the initialization process.

Run qemu with an unattached drive:
  -drive file=$FILE,if=none,id=drive0 \
  -device pcie-root-port,id=rp0,slot=3,bus=pcie.0,hotplug=off
Hotplug a block device:
  device_add virtio-blk-pci,id=blk0,drive=drive0,bus=rp0
If hotplug fails on plug_cb, drive0 will be deleted.

Fixes: 0501e1aa1d32a6 ("hw/pci/pcie: Forbid hot-plug if it's disabled on the slot")
Acked-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Julia Suvorova <jusual@redhat.com>
Message-Id: <20200604125947.881210-1-jusual@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>