Description of problem:
[RFE] Improve the thread model of nbdkit-retry-filter

Version-Release number of selected component (if applicable):
nbdkit-1.28.2-2.el9.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Check the thread model of nbdkit-vddk-plugin and nbdkit-retry-filter

1. nbdkit vddk --dump-plugin | grep thread
   max_thread_model=parallel
   thread_model=parallel

1. nbdkit vddk --filter=retry --dump-plugin | grep thread
   max_thread_model=parallel
   thread_model=serialize_requests

Actual results:
The thread model of retry filter will reduce the thread model of vddk plugin during v2v conversion, details please refer to https://bugzilla.redhat.com/show_bug.cgi?id=2018463#c6

Expected results:
As above description

Additional info: