tests/qemu-iotests: Fix broken grep command in iotest 207

Running "./check -ssh 207" fails for me with lots of lines like this
in the output:

+base64: invalid input

While looking closer at it, I noticed that the grep -v "\\^#" command
in this test is not working as expected - it is likely meant to filter
out the comment lines that are starting with a "#", but at least my
version of grep (GNU grep 3.11) does not work with the backslashes here.
There does not seem to be a compelling reason for these backslashes,
so let's simply drop them to fix this issue.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20251113080525.444826-1-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qemu-iotests/207

@@ -119,7 +119,7 @@ with iotests.FilePath('t.img') as disk_path, \
     iotests.img_info_log(remote_path)
 
     keys = subprocess.check_output(
-        'ssh-keyscan 127.0.0.1 2>/dev/null | grep -v "\\^#" | ' +
+        'ssh-keyscan 127.0.0.1 2>/dev/null | grep -v "^#" | ' +
         'cut -d" " -f3',
         shell=True).rstrip().decode('ascii').split('\n')