qemu: Fix a possible deadlock in p2p migration

The function virUnrefConnect() may call virReleaseConnect() to release
the dest connection, and the function virReleaseConnect() will call
conn->driver->close().

So the function virUnrefConnect() should be surrounded by
qemuDomainObjEnterRemoteWithDriver() and
qemuDomainObjExitRemoteWithDriver() to prevent possible deadlock between
two communicating libvirt daemons.

See commit f0c8e1cb for further details.
Signed-off-by: NWen Congyang <wency@cn.fujitsu.com>

src/qemu/qemu_driver.c

@@ -8790,7 +8790,9 @@ static int doPeer2PeerMigrate(virDomainPtr dom,
 cleanup:
     VIR_FREE(dom_xml);
     /* don't call virConnectClose(), because that resets any pending errors */
+    qemuDomainObjEnterRemoteWithDriver(driver, vm);
     virUnrefConnect(dconn);
+    qemuDomainObjExitRemoteWithDriver(driver, vm);
 
     return ret;
 }