The x86 and armsr targets declare the vdi, vmdk and vhdx features, but
VDI_IMAGES, VMDK_IMAGES and VHDX_IMAGES depend on a hardcoded target
list instead, so the features select nothing and a target opting in has
to edit config/Config-images.in.
Give them USES_VDI, USES_VMDK and USES_VHDX in the same shape as the
other image formats and depend on those. The set of targets offering
each format is unchanged.
Signed-off-by: Daniel Golle <daniel@makrotopia.org>