exec: silence hugetlbfs warning under qtest

vhost-user-test prints a warning. A test should not need to run on
hugetlbfs, let's silence the warning under qtest. The
condition can't check on qtest_enabled() since vhost-user-test actually
doesn't use qtest accel. However, qtest_driver() can be used, if
qtest_init() is called early enough. For that reason, move chardev and
qtest initialization early.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
