tools: hyperv: refactor the default switch code

Rename the method to get_default_external_switch() and modify the PS
command to return only external switches. Introduce a class VMSwitch to
represent a switch. Currently, `name` is the only property and more can
be added later.

Signed-off-by: Anirudh Rayabharam <anrayabh@microsoft.com>

lisa/tools/hyperv.py

@@ -3,9 +3,11 @@
 
 import re
 import time
+from dataclasses import dataclass, field
 from typing import Dict, Optional
 
 from assertpy import assert_that
+from dataclasses_json import config, dataclass_json
 
 from lisa.executable import Tool
 from lisa.operating_system import Windows
@@ -13,6 +15,12 @@ from lisa.tools.powershell import PowerShell
 from lisa.util import LisaException
 
 
+@dataclass_json
+@dataclass
+class VMSwitch:
+    name: str = field(metadata=config(field_name="Name"))
+
+
 class HyperV(Tool):
     # 192.168.5.12
     IP_REGEX = r"\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}"
@@ -156,12 +164,18 @@ class HyperV(Tool):
         if not is_ready:
             raise LisaException(f"VM {name} did not start")
 
-    def get_first_switch(self) -> str:
-        return self.node.tools[PowerShell].run_cmdlet(
-            "Get-VMSwitch | Select -First 1 -ExpandProperty Name",
+    def get_default_external_switch(self) -> Optional[VMSwitch]:
+        switch_json = self.node.tools[PowerShell].run_cmdlet(
+            'Get-VMSwitch | Where-Object {$_.SwitchType -eq "External"} '
+            "| Select -First 1 | select Name | ConvertTo-Json",
             force_run=True,
         )
 
+        if not switch_json:
+            return None
+
+        return VMSwitch.from_json(switch_json)  # type: ignore
+
     def exists_switch(self, name: str) -> bool:
         output = self.node.tools[PowerShell].run_cmdlet(
             f"Get-VMSwitch -Name {name}",