conf: Don't accept dummy values for <memoryBacking/> attributes

conf: Don't accept dummy values for <memoryBacking/> attributes

Our virSomeEnumTypeFromString() functions return either the value
of item from the enum or -1 on error. Usually however the value 0
means 'this value is not set in the domain XML, use some sensible
default'. Therefore, we don't accept corresponding string in
domain XML, for instance:

<memoryBacking>
  <source mode="none"/>
  <access mode="default"/>
  <allocation mode="none"/>
</memoryBacking>

should be rejected as invalid XML.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>