pci: add pci test device

This device is used for kvm unit tests,
currently it supports testing performance of ioeventfd.
Using updated kvm unittest, here's an example output:
        mmio-no-eventfd:pci-mem 8796
        mmio-wildcard-eventfd:pci-mem 3609
        mmio-datamatch-eventfd:pci-mem 3685
        portio-no-eventfd:pci-io 5287
        portio-wildcard-eventfd:pci-io 1762
        portio-datamatch-eventfd:pci-io 1777

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/default-configs/pci.mak b/default-configs/pci.mak
index f5f100e..b608f31 100644
--- a/default-configs/pci.mak
+++ b/default-configs/pci.mak
@@ -24,3 +24,4 @@
 CONFIG_SERIAL_PCI=y
 CONFIG_IPACK=y
 CONFIG_WDT_IB6300ESB=y
+CONFIG_PCI_TESTDEV=y