Only allow ZFS boot on GPT; the MBR ZFS bootblocks cannot be installed using

gpart bootcode as /boot/zfsboot needs to be split into multiple pieces by
hand and copied to different areas of the partition.

usr.sbin/bsdinstall/partedit/partedit_x86.c

@@ -49,7 +49,8 @@ x86_bootmethod(void)
 }
 
 const char *
-default_scheme(void) {
+default_scheme(void)
+{
 	return ("GPT");
 }
 
@@ -77,6 +78,7 @@ is_fs_bootable(const char *part_type, const char *fs)
 		return (1);
 
 	if (strcmp(fs, "freebsd-zfs") == 0 &&
+	    strcmp(part_type, "GPT") == 0 &&
 	    strcmp(x86_bootmethod(), "BIOS") == 0)
 		return (1);