qemu_finish_machine_init currently can only exit QEMU if it fails.
Prepare for giving it proper error propagation, and possibly for
adding a plugin_add monitor command that calls an accelerator
method.

While at it, make all errors from plugin_load look the same.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>