cleanup: make VmDevice.custom ever-present

Device configuration may carry a "custom properties" dictionary that is
copied into its respective VmDevice object. If no such dictionary exist,
the VmDevice.custom is never set.

This patch removes this duality. VmDevice.custom is always there, but it
may be empty if the user has not set any device-specific custom
property.

Change-Id: Icf0677b3d52c3e51c874dcb2b4df12fb3fc22c34
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25391
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>