conf: Report sensible error for invalid disk name

The error "... but the cause is unknown" appeared for XMLs similar to
this:

 <disk type='file' device='cdrom'>
   <driver name='qemu' type='raw'/>
   <source file='/dev/zero'/>
   <target dev='sr0'/>
 </disk>

Notice unsupported disk type (for the driver), but also no address
specified. The first part is not a problem and we should not abort
immediately because of that, but the combination with the address
unknown was causing an unspecified error.

While fixing this, I added an error to one place where this return
value was not managed properly.

src/conf/domain_conf.c

@@ -3055,8 +3055,12 @@ int
 virDomainDiskDefAssignAddress(virCapsPtr caps, virDomainDiskDefPtr def)
 {
     int idx = virDiskNameToIndex(def->dst);
-    if (idx < 0)
+    if (idx < 0) {
+        virReportError(VIR_ERR_XML_ERROR,
+                       _("Unknown disk name '%s' and no address specified"),
+                       def->dst);
         return -1;
+    }
 
     switch (def->bus) {
     case VIR_DOMAIN_DISK_BUS_SCSI:

src/qemu/qemu_command.c

@@ -8381,8 +8381,12 @@ virDomainDefPtr qemuParseCommandLine(virCapsPtr caps,
                 !disk->dst)
                 goto no_memory;
 
-            if (virDomainDiskDefAssignAddress(caps, disk) < 0)
+            if (virDomainDiskDefAssignAddress(caps, disk) < 0) {
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("Cannot assign address for device name '%s'"),
+                               disk->dst);
                 goto error;
+            }
 
             if (VIR_REALLOC_N(def->disks, def->ndisks+1) < 0)
                 goto no_memory;