qemu_tpm.c: fix 'shortName' leak

qemu_tpm.c: fix 'shortName' leak

This is a Coverity fix pointed out by John in IRC. This code
was introduced in 19d74fdf0eb, when the TPM Proxy device for
for ppc64 was introduced.

This will leak in case we have 2 TPMs in the same domain, a
possible scenario with the protected Ultravisor execution in
PowerPC guests.

Fixes: 19d74fdf0eb5d2e89e80ceedea736425160ffccb
Reported-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>