usr.sbin/bhyve: free leaked memory during option parsing

Also update to use strsep(3) instead of strtok(3).

Most of this commit inadvertently ended up in r349914.

Coverity CID:	1357337
Approved by:	markj
PR:		233038
Differential Revision:	https://reviews.freebsd.org/D20918

usr.sbin/bhyve/pci_fbuf.c

@@ -317,7 +317,7 @@ pci_fbuf_parse_opts(struct pci_fbuf_softc *sc, char *opts)
 	}
 
 done:
-	free(uopts);
+	free(uoptsbak);
 	return (ret);
 }