tools: Add virsh iothreadset command

tools: Add virsh iothreadset command

Add a command to allow for setting various dynamic IOThread polling
interval scope (poll-max-ns, poll-grow, and poll-shrink). Describe
the values in the virsh.pod in as generic terms as possible. The
more specific QEMU algorithm has been divulged in the previous patch.

Based heavily on code originally posted by Pavel Hrdina
<phrdina@redhat.com>, but altered to only provide one command
and to not managed a poll disabled state.

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