Instead of having a static AccelType array, register a class for each
accelerator type, and use class name lookup to find accelerator
information.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>