Simplify statistics calculation.

Instead of trying to guess size of disk I/O operations (it just won't work
that way for newly added commands, and is equal to data move size for old
ones), account data move traffic.  If disk I/Os are that interesting, then
backends have to account and provide that information.

Block backend already exports the information about disk I/Os via devstat,
so having it here too is excessive.

MFC after:	2 weeks
- Document -b to enable the bvmcons console (but mark it as deprecated
  similar to -g.)
- Document -U to set the SMBIOS UUID.
- Add missing options to the usage output and to the manpage Synopsis.
- Don't claim that bvmdebug is amd64-only (it is also a device, not an
  option).