kimchi.disks: Ignore unsupported partition table

Some types of partition table such as AIX partition table are used by
proprietary systems. The open-source disk tools such as fdisk and parted
can not handle these partition tables. There is no obvious way to get
extended partition until those proprietary partition tables are
supported by Linux officially.

In this patch we treate unsupported partiton table as if it does not
contain extended partition.

bug-url: https://bugzilla.redhat.com/show_bug.cgi?id=566722

Signed-off-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>