bhyve: remove empty E820 entries

When reserving a block with the same size of a RAM segement, we can end up with
an empty RAM segmenet. Avoid that by removing this empty segment from the E820
table.

Reviewed by: jhb, markj (older version)
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D45480