qemu: Add possibility to prepare top image only for attachment via blockdev

qemu: Add possibility to prepare top image only for attachment via blockdev

qemuBuildStorageSourceChainAttachPrepareBlockdev prepares the full
backing chain for attachment via blockdev. For snapshots we'll need to
prepare one image only as it needs to be plugged on top of the existing
chain.

This patch introduces qemuBuildStorageSourceChainAttachPrepareBlockdevTop
which prepares only @top similarly to the original function by splitting
out the functionality into an internal function so that the API does not
change.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>