qemu: Introduce qemuDomainSetIOThreadParams

qemu: Introduce qemuDomainSetIOThreadParams

https://bugzilla.redhat.com/show_bug.cgi?id=1545732

Implement the QEMU driver mechanism in order to set the polling
parameters for an IOThread within the bounds specified by the
QEMU qapi parameter passing.

Based heavily on patches originally posted by Pavel Hrdina
<phrdina@redhat.com>, but modified to only handle alterations
for a running guest. For the most part the API names changed,
the typed parameters removed the poll enabled value, and the
capabilities check was moved to just before the live attempt
to set. Since changes are only supported for a running guest,
no guest XML alterations were kept.

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