bhyve: avoid an empty passthru config value

[corvink]

Summary

pci_parse_legacy_config splits the options string by comma characters.
strchr returns a pointer to the first occurence of a character. In that
case, it's a comma. So, pci_parse_legacy_config will stop at the first
character and creates a new config node with a name of NULL.

[jhb]

usr.sbin/bhyve/pci_passthru.c

670