bhyve: enumerate BARs by size

E.g. Framebuffers can require large space and BARs need to be aligned
by their size. If BARs aren't allocated by size, it'll cause much
fragmentation of the MMIO space. Reduce fragmentation by ordering
the BAR allocation on their size to reduce the risk of
OUT_OF_MMIO_SPACE issues.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D28278