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: NMichal Privoznik <mprivozn@redhat.com>

src/conf/domain_conf.c

@@ -16724,7 +16724,7 @@ virDomainDefParseXML(xmlDocPtr xml,
 
     tmp = virXPathString("string(./memoryBacking/source/@type)", ctxt);
     if (tmp) {
-        if ((def->mem.source = virDomainMemorySourceTypeFromString(tmp)) < 0) {
+        if ((def->mem.source = virDomainMemorySourceTypeFromString(tmp)) <= 0) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                            _("unknown memoryBacking/source/type '%s'"), tmp);
             goto error;
@@ -16734,7 +16734,7 @@ virDomainDefParseXML(xmlDocPtr xml,
 
     tmp = virXPathString("string(./memoryBacking/access/@mode)", ctxt);
     if (tmp) {
-        if ((def->mem.access = virDomainMemoryAccessTypeFromString(tmp)) < 0) {
+        if ((def->mem.access = virDomainMemoryAccessTypeFromString(tmp)) <= 0) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                            _("unknown memoryBacking/access/mode '%s'"), tmp);
             goto error;
@@ -16744,7 +16744,7 @@ virDomainDefParseXML(xmlDocPtr xml,
 
     tmp = virXPathString("string(./memoryBacking/allocation/@mode)", ctxt);
     if (tmp) {
-        if ((def->mem.allocation = virDomainMemoryAllocationTypeFromString(tmp)) < 0) {
+        if ((def->mem.allocation = virDomainMemoryAllocationTypeFromString(tmp)) <= 0) {
             virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                            _("unknown memoryBacking/allocation/mode '%s'"), tmp);
             goto error;