qemu_security: Fully implement qemuSecurityDomainSetPathLabel

qemu_security: Fully implement qemuSecurityDomainSetPathLabel

Even though the current use of the function does not require full
implementation with transactions (none of the callers pass a path
somewhere under /dev), it doesn't hurt either. Moreover, in
future patches the paradigm is going to shift so that any API
that touches a file is required to use transactions.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>