lib: Introduce virDomainSetIOThreadParams

lib: Introduce virDomainSetIOThreadParams

Create a new API that will allow an adjustment of IOThread
polling parameters for the specified IOThread. These parameters
will not be saved in the guest XML. Currently the only parameters
supported will allow the hypervisor to adjust the parameters used
to limit and alter the scope of the polling interval. The polling
interval allows the IOThread to spend more or less time processing
in the guest.

Based on code originally posted by Pavel Hrdina <phrdina@redhat.com>
to add virDomainAddIOThreadParams and virDomainModIOThreadParams.
Modification of those changes to use virDomainSetIOThreadParams
instead and remove concepts related to saving the data in guest
XML as well as the way to specifically enable the polling parameters.

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Michal Privoznik <mprivozn@redhat.com>