vmdk: push vmdk_read_desc up to caller

vmdk: push vmdk_read_desc up to caller

Currently, we just try reading a VMDK file as both image and descriptor.
This makes it hard to choose which of the two attempts gave the best error.
We'll decide in advance if the file looks like an image or a descriptor,
and this patch is the first step to that end.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>