snapshot_disk: Fix block disk and raw disk external snapshot problem

When attach raw disk and use xml to do snapshot, update driver type
to qcow2 as it's the only support external snapshot format.

When domain disk is 'block' type, update disk type to 'file' and
source file to external snapshot file in snapshot xml.

Also, use param export_options to avoid nfs problem on fedora.

Signed-off-by: Wayne Sun <gsun@redhat.com>