bootstrap: move asset functions to a separate library

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
image_copy: use assets as the source

Use the asset library to download images if the images_good parameter
is not set.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
image_copy: skip extraction in image_copy if the image is found to be ok

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>