vdi: Fix .bdrv_has_zero_init()

vdi: Fix .bdrv_has_zero_init()

Static VDI images cannot guarantee to be zero-initialized.  If the image
has been statically allocated, forward the call to the underlying
storage node.

Reported-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190724171239.8764-8-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>