qemuMigrationSrcIsSafeDisk: Extract safe migration checks for one storage source
Further split up the code originally in 'qemuMigrationSrcIsSafe' to
separate checks concerning a single storage source.
The code will then be reused to check the safe migration state also for
the data file (qcow2 feature that allows store of data separated from
the qcow2 metadata).
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>