Support GET-only HTTP servers for image sources (#472)

Some HTTP services, notably GitHub Releases, do not support HEAD requests to their endpoints.

terraform-provider-libvirt will thus fail on something along the lines of:

resource "libvirt_volume" "centos7_volume" {
  name = "centos7"
  source = "https://github.com/moio/sumaform-images/releases/download/4.0.0/centos7.qcow2"
}

With:

libvirt_volume.centos7_volume: Error accessing remote resource: https://github.com/moio/sumaform-images/releases/download/4.0.0/centos7.qcow2 - 403 Forbidden


This removes the assumption a HEAD request will succeed. If it doesn't, it will try a Body-less GET instead.

libvirt/utils_volume.go

@@ -94,6 +94,15 @@ func (i *httpImage) Size() (uint64, error) {
 	if err != nil {
 		return 0, err
 	}
+	if response.StatusCode == 403 {
+		// possibly only the HEAD method is forbidden, try a Body-less GET instead
+		response, err = http.Get(i.url.String())
+		if err != nil {
+			return 0, err
+		}
+
+		response.Body.Close()
+	}
 	if response.StatusCode != 200 {
 		return 0,
 			fmt.Errorf(