scripts: improve error from qemu-trace-stap on missing 'stap'

If the 'stap' binary is missing in $PATH, a huge trace is thrown

  $ qemu-trace-stap list /usr/bin/qemu-system-x86_64
  Traceback (most recent call last):
  File "/usr/bin/qemu-trace-stap", line 169, in <module>
  main()
  File "/usr/bin/qemu-trace-stap", line 165, in main
  args.func(args)
  File "/usr/bin/qemu-trace-stap", line 83, in cmd_run
  subprocess.call(stapargs)
  File "/usr/lib64/python3.12/subprocess.py", line 389, in call
  with Popen(*popenargs, **kwargs) as p:
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/subprocess.py", line 1026, in {}init{}
  self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib64/python3.12/subprocess.py", line 1955, in _execute_child
  raise child_exception_type(errno_num, err_msg, err_filename)
  FileNotFoundError: [Errno 2] No such file or directory: 'stap'

With this change the user now gets

  $ qemu-trace-stap list /usr/bin/qemu-system-x86_64
  Unable to find 'stap' in $PATH

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20241206114524.1666664-1-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/qemu-trace-stap

@@ -56,6 +56,7 @@ def tapset_dir(binary):
 
 
 def cmd_run(args):
+    stap = which("stap")
     prefix = probe_prefix(args.binary)
     tapsets = tapset_dir(args.binary)
 
@@ -76,7 +77,7 @@ def cmd_run(args):
 
     # We request an 8MB buffer, since the stap default 1MB buffer
     # can be easily overflowed by frequently firing QEMU traces
-    stapargs = ["stap", "-s", "8", "-I", tapsets ]
+    stapargs = [stap, "-s", "8", "-I", tapsets ]
     if args.pid is not None:
         stapargs.extend(["-x", args.pid])
     stapargs.extend(["-e", script])
@@ -84,6 +85,7 @@ def cmd_run(args):
 
 
 def cmd_list(args):
+    stap = which("stap")
     tapsets = tapset_dir(args.binary)
 
     if args.verbose:
@@ -96,7 +98,7 @@ def cmd_list(args):
 
         if verbose:
             print("Listing probes with name '%s'" % script)
-        proc = subprocess.Popen(["stap", "-I", tapsets, "-l", script],
+        proc = subprocess.Popen([stap, "-I", tapsets, "-l", script],
                                 stdout=subprocess.PIPE,
                                 universal_newlines=True)
         out, err = proc.communicate()