Fix several logic issues in domainset_empty_vm().

• Don't add 1 to the result of DOMAINSET_FLS.
• Do not modify domainsets containing only empty domains.
• Always flatten a _PREFER policy to _ROUNDROBIN if the preferred domain is empty. Previously we were doing this only when ds_cnt > 1.

These bugs could cause hangs during boot if a VM domain is empty.

Tested by: hselasky
Reviewed by: hselasky, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21420