virDomainVcpuParse: Assign default vcpus count based on return value of virXMLPropUInt

virDomainVcpuParse: Assign default vcpus count based on return value of virXMLPropUInt

Assign the vcpu count when virXMLPropUInt returns '0' meaning that the
cpu count was not present in the XML. This will allow to always
initialize the value of @result in virXMLPropUInt to prevent use of
uninitialized values.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>