test_driver: Implement virDomainGetIOThreadInfo

test_driver: Implement virDomainGetIOThreadInfo

If we use test driver on different machines, and use 0 as bitmap_size
for virDomainDriverGetIOThreadsConfig(), we would get different results for
the `CPU Affinity`, because it's depending on the host CPU's bitmap. In
order to get a stable result for testing, use result of
virDomainDefGetVcpus() as bitmap_size instead.

Signed-off-by: Luke Yue <lukedyue@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>