hw/net: report errors from failing to use eBPF RSS FDs

hw/net: report errors from failing to use eBPF RSS FDs

If the user/mgmt app passed in a set of pre-opened FDs for eBPF RSS,
then it is expecting QEMU to use them. Any failure to do so must be
considered a fatal error and propagated back up the stack, otherwise
deployment mistakes will not be detectable in a prompt manner. When
not using pre-opened FDs, then eBPF RSS is tried on a "best effort"
basis only and thus fallback to software RSS is valid.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>