Vivek Goyal found out that using "shared" thread pool, instead of
"exclusive" results in better performance.
Knowning that and with the plan to have virtio-fs as the default fs for
the 2.0, let's bring this patch in for both 5.0 and 5.1.
Fixes: #944
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>