spice: don't force user to specify spicevmc channel

We support only one spicevmc channel name anyway and the code is
prepared to use the default one, there's only one check missing.  It
is also mentioned in the documentation already and helps defining
domains with spice vdagent for people using virsh.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/qemu/qemu_command.c

@@ -6134,7 +6134,8 @@ qemuBuildVirtioSerialPortDevStr(virDomainChrDefPtr dev,
 
     if (dev->deviceType == VIR_DOMAIN_CHR_DEVICE_TYPE_CHANNEL &&
         dev->source.type == VIR_DOMAIN_CHR_TYPE_SPICEVMC &&
-        STRNEQ_NULLABLE(dev->target.name, "com.redhat.spice.0")) {
+        dev->target.name &&
+        STRNEQ(dev->target.name, "com.redhat.spice.0")) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
                        _("Unsupported spicevmc target name '%s'"),
                        dev->target.name);