qemu: Use @tmpChr in qemuDomainDetachChrDevice to build device string

So far we are passing @chr to qemuBuildChrDeviceStr. This is
suboptimal (in fact wrong) because @chr is just parsed XML
definition provided by user which by definition may lack some
information. On the other hand, @tmpChr is the one that was found
using @chr in domain definition so it contains the same amount of
information or more.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_hotplug.c

@@ -6131,7 +6131,7 @@ int qemuDomainDetachChrDevice(virQEMUDriverPtr driver,
 
     sa_assert(tmpChr->info.alias);
 
-    if (qemuBuildChrDeviceStr(&devstr, vmdef, chr, priv->qemuCaps) < 0)
+    if (qemuBuildChrDeviceStr(&devstr, vmdef, tmpChr, priv->qemuCaps) < 0)
         goto cleanup;
 
     if (!async)