qemu: Fix double free in qemuDomainSecretAESClear

Commit id 02b031a4 added a secondary path from which the
incoming @secinfo would not be free'd until the private
data was freed in qemuDomainStorageSourcePrivateDispose.

However, by doing this the original intention to free
@*secinfo afterwards is lost and thus the pass by value
of the secinfo->s.aes (or secinfo->s.plain for its method)
results in not keeping the NULL setting in the various
secret.{username|iv|ciphertext} fields upon return to
qemuDomainSecretInfoClear and eventually will result in
a double free at domain destroy:

raise ()
abort ()
__libc_message ()
malloc_printerr ()
_int_free ()
virFree
qemuDomainSecretAESClear
qemuDomainSecretInfoClear
qemuDomainSecretInfoFree
qemuDomainStorageSourcePrivateDispose
virObjectUnref
virStorageSourceClear
virStorageSourceFree
virDomainDiskDefFree
virDomainDefFree
virDomainObjRemoveTransientDef
qemuProcessStop
qemuDomainDestroyFlags
virDomainDestroy

Signed-off-by: John Ferlan <jferlan@redhat.com>
ACKed-by: Peter Krempa <pkrempa@redhat.com>