esp: Remove user_creatable flag

esp devices aren't going to work with -device, as they need IRQs
to be connected and mmio to be mapped (this is done by
esp_init()). Remove the user_creatable flag from the device
class.

Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170503203604.31462-16-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>