qemu: Handle host devices not being available better

qemu: Handle host devices not being available better

We can't retrieve the isolation group of a device that's not present
in the system. However, it's very common for VFs to be created late
in the boot, so they might not be present yet when libvirtd starts,
which would cause the guests using them to disappear.

Moreover, for other architectures and even ppc64 before isolation
groups were introduced, it's considered perfectly fine to configure a
guest to use a device that's not yet (or no longer) available to the
host, with the obvious caveat that such a guest won't be able to
start before the device is available.

In order to be consistent, when a device's isolation group can't be
determined fall back to not isolating it rather than erroring out or,
worse, making the guest disappear.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1484254

Signed-off-by: Andrea Bolognani <abologna@redhat.com>