Issue 292 Logical Storage Pool Returning "extended" Partitions as Possible Pool

Extended partition is a container of logical partitions. Kimchi should
skip extended partition regardless of it contains logical partitions or
not. This patch uses pyparted to check if a partition is a extended
partition and skip if it is.

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