bhyve: avoid an empty passthru config value

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.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D34600

(cherry picked from commit 3256b7ca36e739a08f141a7ee52b9db9d995db85)