kvm: add API to set ioeventfd

Comment on kvm usage: rather than require users to do if (kvm_enabled())
and/or ifdefs, this patch adds an API that, internally, is defined to
stub function on non-kvm build, and checks kvm_enabled for non-kvm
run.

While rest of qemu code still uses if (kvm_enabled()), I think this
approach is cleaner, and we should convert rest of code to it
long term.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
