qemuMigrationSrcIsSafeDisk: Allow non-shared qcow2's with raw data file

qemuMigrationSrcIsSafeDisk: Allow non-shared qcow2's with raw data file

A qcow2 image which uses a data file and the 'data_file_raw' flag is
effectively a raw image with the qcow2 wrapper used only to store
metadata (block dirty bitmaps).

Since the dirty bitmaps are always migrated using the migration stream
it's technically not required that the qcow2 overlay itself is shared
between the destinations.

Management tools like Kubevirt want to migrate VMs which have a qcow2
overlay with the above config stored in a location that is not shared,
but the data file itself is.

This patch adds code that skips the validation of the overlay since it's
not needed to ensure data consistency in that very specific case.

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