virsystemd: Fix error check

virsystemd: Fix error check

Commit b3e4401dc620 introduced a check to ignore an error if the guest
is already terminated. However the check accidentally compared
error.code with VIR_ERR_ERROR, which is an error level, not an error
code. Because of this, almost every error got silently ignored.

Fixes: b3e4401dc620 ("systemd: don't report an error if the guest is
already terminated")

Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>