virbuffer: Use signed integer for storing error

The @error member can contain a positive value (errno) or a
negative value (-1) to denote a usage error. It doesn't make
much sense to store it as unsigned then.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

src/util/virbuffer.h

@@ -40,7 +40,7 @@ typedef virBuffer *virBufferPtr;
 struct _virBuffer {
     size_t size;
     size_t use;
-    unsigned int error; /* errno value, or -1 for usage error */
+    int error; /* errno value, or -1 for usage error */
     int indent;
     char *content;
 };