Add new image type:vdh support in bootc image builder

vdh is newly introduced support disk image type, and is only supported
on upstream

Signed-off-by: chunfuwen <chwen@redhat.com>

virttools/tests/cfg/bootc_image_builder/bootc_disk_image_build.cfg

@@ -158,3 +158,8 @@
             disk_image_type = "raw"
             output_sub_folder = "image"
             output_name = "disk.raw"
+        - vhd:
+            disk_image_type = "vhd"
+            output_sub_folder = "vpc"
+            output_name = "disk.vhd"
+            only upstream_bib