bhyve: don't panic if e820 finds no available memory

The GVT-d emulation tries to allocate some specific memory. It could
happen that this address doesn't exist. In that case, GVT-d will fall
back to allocate any address. Nevertheless, this only works if the e820
fails with an error instead of exiting on an assertion.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40034