webadmin: fix pool creation

This patch fixes pool creation which broke by the latest changes related
to graphic devices. We tried to add a value to the values collection of
a Map instead of adding it using Map#put method.

Change-Id: Iddfeeb01835de7116d91201a1f81b40f7a63fe2e
Signed-off-by: Arik Hadas <ahadas@redhat.com>