vmdk: To add missing time unit in driver option

Time unit is missing in the description of vmdk driver option
"vmware_task_poll_interval". Due to this, the vmdk driver config
reference document doesn't have the timeunit in its description
column. This fix adds the missing time unit (seconds) in the
driver option.

Closes-Bug: #1258100
Change-Id: I2c49c65ca9a48a2dcc6898a32b4bbc0433e8916b
Merge "vmdk: To add missing time unit in driver option"