qemu: Detect whether iothread polling is supported

qemu: Detect whether iothread polling is supported

Add a capability check for IOThread polling (all were added at the
same time, so only one check is necessary).

Based on code originally posted by Pavel Hrdina <phrdina@redhat.com>
with the only changes to include the more recent QEMU releases.

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