nanoBSD boot problem (on USB stick or as a HD)

Hi all,
I created a nanoBSD image for my gsoc project (ptnetmap on bhyve).

I would like to boot this image on USB stick or in the hypervisor as a HD.
I have some problem because if I set NANO_DRIVE="da0" (for USB boot)
in the nanoBSD configuration file, the boot from USB stick works well,
but when I try to boot the same image in the hypervisor as a HD,
I have the following mountroot error:

Trying to mount root from ufs:/dev/da0s1a [ro]...
mountroot: waiting for device /dev/da0s1a ...
Mounting from ufs:/dev/da0s1a failed with error 19.

Loader variables:
   vfs.root.mountfrom=ufs:/dev/da0s1a
   vfs.root.mountfrom.options=ro

mountroot>


At this point I need to manually specify "ufs:/dev/ad0s1a" to properly mount
the root.

Can you help me?
There is some tricks to avoid this mountroot error?

Thanks,
Stefano Garzarella