fix(instances): find known VMs by UUID, not by name

Instance.proxy looked the domain up by its stored name. After a rename
or a name swap outside WebVirtCloud, an operation on one VM changed
another; after a VM was replaced under the same name, operations hit
the new VM.

wvmInstance takes the UUID and uses lookupByUUIDString, with no
fallback to the name: a missing UUID fails instead of picking another
domain. Every caller that has an Instance passes it: proxy, console,
quota, post-migration autostart and the VDI URL. Migration uses the
domain the proxy already resolved. getvvfile and the gstfsd requests
use the host's current name of that UUID.

Live tests cover a name swap and a VM replaced under the same name;
both fail on the previous code.

vrtManager/instance.py

@@ -126,7 +126,7 @@ class wvmInstances(wvmConnect):
         if undefine:
             flags |= VIR_MIGRATE_UNDEFINE_SOURCE
 
-        dom = conn.get_instance(name)
+        dom = conn.instance
 
         dom_arch = conn.get_arch()
         dom_emulator = conn.get_dom_emulator()
@@ -177,10 +177,12 @@ class wvmInstances(wvmConnect):
 
 
 class wvmInstance(wvmConnect):
-    def __init__(self, host, login, passwd, conn, vname):
+    def __init__(self, host, login, passwd, conn, vname, uuid=None):
         wvmConnect.__init__(self, host, login, passwd, conn)
         self._ip_cache = None
-        self.instance = self.get_instance(vname)
+        # A known VM is found by UUID: a name can be renamed or swapped
+        # outside WebVirtCloud. No fallback to the name.
+        self.instance = self.wvm.lookupByUUIDString(uuid) if uuid else self.get_instance(vname)
 
     def osinfo(self):
         info_results = qemuAgentCommand(