Dedicated ISO pool: create an out of box ISO pool

An out of box ISO pool is created to make sure user
will find a well known place to put his ISO.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Store qemu user name in class attribute

To prevent qemu user be probed multiple times,
store it in class attribute so that next time we don't need to create vm.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>
Add doc and test case for dedicate iso pool

Add doc for user to upload the iso to a dedicate pool,
add testcase to validate correctness.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>