igd-passthrough: fix use of host_pci_config_read

Fix the bug introduced by 595a4f07: function host_pci_config_read() should be
pass-by-reference, not value.
This probably means this function never worked for anyone.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>