qemu_security: Rework qemuSecurityCleanupTPMEmulator()

qemu_security: Rework qemuSecurityCleanupTPMEmulator()

Currently, qemuSecurityCleanupTPMEmulator() returns nothing which
means a caller (well, there's only one - qemuExtTPMStop()) can't
produce a warning when restoring seclabels on TPM state failed.
True, qemuSecurityCleanupTPMEmulator() does report a warning
itself, but only in one specific error path.

Make the function return an integer, just like the rest of
qemuSecurity*Restore() functions.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>