qemuMigrationDstPrecreateStorage: Fix and clarify logic

qemuMigrationDstPrecreateStorage: Fix and clarify logic

While it's intended that qemuMigrationDstPrecreateDisk is called with
any kind of the disk, the logic in qemuMigrationDstPrecreateStorage
which checks the existence of the image wouldn't properly handle e.g.
network backed disks, where it would attempt to use virFileExists() on
the disk's 'src->path'.

Fix the logic by first skipping disks not meant for migration, then do
the existence check only when 'disk->src' is local storage.

Since qemuMigrationDstPrecreateDisk has a debug statement there's no
need to have an extra one right before calling into it.

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