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

[corvink]

Summary

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.

[markj]

usr.sbin/bhyve/e820.c

199

Please add a comment describing the scenario where this happens.

[corvink]

• add comment to explain when adding an e820 entry is intended to fail