Of the block device commands, those that are available outside system
emulators do not require a fully constructed machine by definition.
Allow running them before machine initialization has concluded.

Of the ones that are available inside system emulation, allow querying
the PR managers, and setting up accounting and throttling.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>