vhost-user-test: extract read-guest-mem test from main loop

This patch makes read-guest-test consistent with other tests,
i.e. create the test server in the test function.

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