distro: define PartitionType method on ImageType

This is needed so that we can do different things depending on the 
given layout; this will be used in tests for now only. Only GPT
allows for arbitrary number of partitions and once we assert this
in code we will need to adjust the tests accordingly.

NB: This method might be removed again in the future, once generic
LVM support is added everywhere and the ability to differentiate 
between MBR and GPT layouts is not needed anymore.

internal/distro/test_distro/distro.go

@@ -167,6 +167,10 @@ func (t *TestImageType) Size(size uint64) uint64 {
 	return 0
 }
 
+func (t *TestImageType) PartitionType() string {
+	return ""
+}
+
 func (t *TestImageType) Packages(bp blueprint.Blueprint) ([]string, []string) {
 	return nil, nil
 }