Pool.volumes property is implemented in a base class, individual drivers
should provide list_volumes() method as a backend for that property.
Fix this in a LinuxKernel pool.
Pool setup/destroy may be a time consuming operation, allow them to be
asynchronous. Fortunately add_pool and remove_pool are used only through
Admin API, so the change does not require modification of other
components.
Some kernels (like pvgrub2) may not provide modules.img and it isn't an
error. Don't break VM startup in that case, skip that device instead.
FixesQubesOS/qubes-issues#3563
