qemu: Drop default channel path during migration

Migration to an older libvirt (pre v1.3.0-175-g71408079) is broken
because older versions of libvirt generated different channel paths and
they didn't drop the default paths when parsing domain XMLs. We'd get
such a nice error message:

    internal error: process exited while connecting to monitor:
    2016-07-08T15:28:02.665706Z qemu-kvm: -chardev socket,
    id=charchannel0,path=/var/lib/libvirt/qemu/channel/target/
    domain-3-nest/org.qemu.guest_agent.0,server,nowait: Failed to bind
    socket to /var/lib/libvirt/qemu/channel/target/domain-3-nest/
    org.qemu.guest_agent.0: No such file or directory

That said, we should not even format the default paths when generating a
migratable XML.

https://bugzilla.redhat.com/show_bug.cgi?id=1320470Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

src/qemu/qemu_domain.c

@@ -2360,6 +2360,29 @@ qemuDomainDefaultNetModel(const virDomainDef *def,
     return "rtl8139";
 }
 
+
+/*
+ * Clear auto generated unix socket path, i.e., the one which starts with our
+ * channel directory.
+ */
+static void
+qemuDomainChrDefDropDefaultPath(virDomainChrDefPtr chr,
+                                virQEMUDriverPtr driver)
+{
+    virQEMUDriverConfigPtr cfg = virQEMUDriverGetConfig(driver);
+
+    if (chr->deviceType == VIR_DOMAIN_CHR_DEVICE_TYPE_CHANNEL &&
+        chr->targetType == VIR_DOMAIN_CHR_CHANNEL_TARGET_TYPE_VIRTIO &&
+        chr->source.type == VIR_DOMAIN_CHR_TYPE_UNIX &&
+        chr->source.data.nix.path &&
+        STRPREFIX(chr->source.data.nix.path, cfg->channelTargetDir)) {
+        VIR_FREE(chr->source.data.nix.path);
+    }
+
+    virObjectUnref(cfg);
+}
+
+
 static int
 qemuDomainDeviceDefPostParse(virDomainDeviceDefPtr dev,
                              const virDomainDef *def,
@@ -2441,21 +2464,8 @@ qemuDomainDeviceDefPostParse(virDomainDeviceDefPtr dev,
 
     /* clear auto generated unix socket path for inactive definitions */
     if ((parseFlags & VIR_DOMAIN_DEF_PARSE_INACTIVE) &&
-        dev->type == VIR_DOMAIN_DEVICE_CHR &&
-        dev->data.chr->deviceType == VIR_DOMAIN_CHR_DEVICE_TYPE_CHANNEL &&
-        dev->data.chr->targetType == VIR_DOMAIN_CHR_CHANNEL_TARGET_TYPE_VIRTIO &&
-        dev->data.chr->source.type == VIR_DOMAIN_CHR_TYPE_UNIX &&
-        dev->data.chr->source.data.nix.path &&
-        STRPREFIX(dev->data.chr->source.data.nix.path, cfg->channelTargetDir)) {
-        /*
-         * If the address is generated by us (starts with our
-         * channel dir), we should not keep it in the persistent
-         * XML.  If libvirt is the one who generated it, users
-         * shouldn't care about that.  If they do, they are
-         * supposed to set it themselves.
-         */
-        VIR_FREE(dev->data.chr->source.data.nix.path);
-    }
+        dev->type == VIR_DOMAIN_DEVICE_CHR)
+        qemuDomainChrDefDropDefaultPath(dev->data.chr, driver);
 
     /* forbid capabilities mode hostdev in this kind of hypervisor */
     if (dev->type == VIR_DOMAIN_DEVICE_HOSTDEV &&
@@ -3270,7 +3280,8 @@ qemuDomainDefFormatBuf(virQEMUDriverPtr driver,
             virDomainControllerDefFree(usb);
         }
 
-
+        for (i = 0; i < def->nchannels; i++)
+            qemuDomainChrDefDropDefaultPath(def->channels[i], driver);
     }
 
  format: