conf: Turn def->maxvcpus into size_t

Later on this will also be used to track size of the vcpu data array.
Use size_t so that we can utilize the memory allocation helpers.

src/conf/domain_conf.c

@@ -1317,7 +1317,7 @@ virDomainDefSetVcpus(virDomainDefPtr def,
 {
     if (vcpus > def->maxvcpus) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                       _("maxvcpus must not be less than current vcpus (%u < %u)"),
+                       _("maxvcpus must not be less than current vcpus (%u < %zu)"),
                        vcpus, def->maxvcpus);
         return -1;
     }

src/conf/domain_conf.h

@@ -2219,7 +2219,7 @@ struct _virDomainDef {
     virDomainMemtune mem;
 
     unsigned int vcpus;
-    unsigned int maxvcpus;
+    size_t maxvcpus;
     int placement_mode;
     virBitmapPtr cpumask;