core,webadmin: Plug disk to VM when adding a new disk

When adding a new disk to a VM the disk is plugged upon creation only if
the VM is down, this patch is plugging the disk by default after the disk
creation is completed even if the VM is not down, this behavior can be
overriden by specifing a parameter to the add disk command, the new disk
popup in webadmin was changed to support this behavior

Bug-Url: https://bugzilla.redhat.com/856272
Change-Id: Ia9778bcaf21b346a55992590159cabd8d78f0c66
Signed-off-by: Tal Nisan <tnisan@redhat.com>