xenstored: Default priv_domid to store_domid

On a system without a control domain, a hardware|xenstore domain
starting xenstored shows:
common/grant_table.c:1041:d2v0 Could not find domain 32756
gnttab: error: mmap failed: Invalid argument
Could not initialize dom32756

setup_structure() does not get called without priv_domid.  Subsequent
introduce_domain() calls will fault when calling fire_special_watches().

Default priv_domid to store_domid when there is no control domain.

Fixes: e5b0a94055 ("tools/xenstored: Auto-introduce domains")
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

tools/xenstored/domain.c

@@ -1388,6 +1388,12 @@ void init_domains(void)
 	if (store_domid == DOMID_INVALID)
 		barf("Could not determine xenstore domid\n");
 
+	if (priv_domid == DOMID_INVALID)
+	{
+		priv_domid = store_domid;
+		xprintf("priv_domid defaulted to %u\n", priv_domid);
+	}
+
 	snprintf(store_domain_path, sizeof(store_domain_path),
 		 "/local/domain/%u", store_domid);