MFC r256389

  Implement the virtio block 'get-ident' operation. This eliminates the
  annoying verbose boot error of the form

     g_handleattr: vtbd0 bio_length 24 len 28 -> EFAULT

  The ident returned by bhyve is a text string 'BHYVE-XXXX-XXXX', where
  the X's are the first bytes of the md5 hash of the backing filename.

Approved by:	re (gjb)
MFC r256391:
Fix signed integer overflow detection in f_expand_number() of strings.subr.

Approved by:	re (glebius)
Update stable/10 to -BETA1 as part of the 10.0-RELEASE cycle.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation