UI bug fix: Properly display storage volumes on Storage tab

While selecting an active storage pool, the storage volumes in it should
be listed.
But commit 1fb4257f changed "hide-content" CSS class to "display: none!important"
which is preveting the storage volumes to be displayed through
.slideDown() function.
Set "style=display:none" to the volumes div to fix it.

Signed-off-by: Aline Manera <alinefm@linux.vnet.ibm.com>