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: NMichael S. Tsirkin <mst@redhat.com>