userportal: Edit VM on UP fails on NPE

The regression has been introduced by the SerialNumber feature:
72fcf57dc92d4e5469b1911cb5b3b42e433bd784

which adds the possibility to edit the serial number also to the UP but the
query which is supposed to return, if the serial number is supported for the
specific version is not a user query leading to a NPE in AbstractVmPopupWidget
where the isSerialNumberPolicySupported was being converted to boolean. And
since it was null, this conversion failed on NPE.

Change-Id: Ifdd13ded70e3b4abb17c114ebbe86b11a3ef31ea
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>