virtio-pci: use ioeventfd even when KVM is disabled

virtio-pci: use ioeventfd even when KVM is disabled

Old kvm.ko versions only supported a tiny number of ioeventfds so
virtio-pci avoids ioeventfds when kvm_has_many_ioeventfds() returns 0.

Do not check kvm_has_many_ioeventfds() when KVM is disabled since it
always returns 0.  Since commit 8c56c1a592b5092d91da8d8943c17777d6462a6f
("memory: emulate ioeventfd") it has been possible to use ioeventfds in
qtest or TCG mode.

This patch makes -device virtio-blk-pci,iothread=iothread0 work even
when KVM is disabled.

I have tested that virtio-blk-pci works under TCG both with and without
iothread.

This patch fixes qemu-iotests 068, which was accidentally merged early
despite the dependency on ioeventfd.

Cc: Michael S. Tsirkin <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Tested-by: Eric Blake <[email protected]>
Tested-by: Kevin Wolf <[email protected]>
Message-id: 20170628184724[email protected]
Message-id: 20170615163813[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>