qemu_validate: Check if QEMU's capable of setting iothread pool size
qemu_validate: Check if QEMU's capable of setting iothread pool size

Now that we have a capability that reflects whether QEMU is
capable of setting iothread pool size, let's introduce a
validator check to make sure users are not trying to use this
feature with QEMU that doesn't support it.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>

zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,olafhering/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,libvirt/libvirt