Issue #290: correct the log usage

It is recommended that the log message should use comma.
Arguments should be separated by comma.
and they should not be in a tuple.

Before this patch:
Kimchi throws an exception as follow:
TypeError: not enough arguments for format string
Logged from file disks.py, line 117

After use the log correctly:
Kimchi log works well:
Error getting partition info for 35000c5002eaa8392p2: Error executing
lsblk: lsblk: dm-2: unknown device name

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>