qemu_vm.py: update vhost thread matching pattern and process command

- updated the vhost_thread_pattern to match vhost thread names with
or without square brackets
- switched to using "ps -eL" for vhost thread detection, as vhost
threads are now created as separate threads and no longer appear in
"ps aux" output in the latest kernel. "ps -eL" also works with older
kernels.

Signed-off-by: Wenli Quan <[email protected]>

virttest/qemu_vm.py

@@ -3945,7 +3945,7 @@ def create(
                     ", ".join([str(tid) for tid in self.vcpu_threads]),
                 )
             vhost_thread_pattern = params.get(
-                "vhost_thread_pattern", r"\w+\s+(\d+)\s.*\[vhost-%s\]"
+                "vhost_thread_pattern", r"(\d+)\s+\d+\s.*vhost-%s"
             )
             self.vhost_threads = self.get_vhost_threads(vhost_thread_pattern)
 
@@ -4396,7 +4396,7 @@ def get_vhost_threads(self, vhost_thread_pattern):
             int(_)
             for _ in re.findall(
                 vhost_thread_pattern % self.get_pid(),
-                process.run("ps aux", verbose=False).stdout_text,
+                process.run("ps -eL", verbose=False).stdout_text,
             )
         ]