ebpf: add formal error reporting to all APIs

ebpf: add formal error reporting to all APIs

The eBPF code is currently reporting error messages through trace
events. Trace events are fine for debugging, but they are not to be
considered the primary error reporting mechanism, as their output
is inaccessible to callers.

This adds an "Error **errp" parameter to all methods which have
important error scenarios to report to the caller.

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