Use zfs_read() instead of xfsread() to read /boot.config. xfsread() fails

short read requests, so the result was that a /boot.config smaller than 512
bytes was ignored.  boot2 uses fsread() instead of xfsread() to read
/boot.config already, so this makes zfsboot more like boot2.

Submitted by:	Johny Mattsson  johny-freebsd of earthmagic org
Reviewed by:	dfr
MFC after:	3 days

sys/boot/i386/zfsboot/zfsboot.c

@@ -609,7 +609,7 @@ main(void)
 
     if (zfs_lookup(spa, PATH_CONFIG, &dn) == 0) {
 	off = 0;
-	xfsread(&dn, &off, cmd, sizeof(cmd));
+	zfs_read(spa, &dn, &off, cmd, sizeof(cmd));
     }
 
     if (*cmd) {