qcow2: Cleanups and memleak fix in qcow2_snapshot_create

sn->id_str could be leaked before this. The rest of this patch changes
comments, fixes coding style or removes checks that are unnecessary with
g_malloc.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
