vhost-user-test: create a temporary directory per TestServer

This makes the tests more independent, and also the source and destination
TestServers in the migration test.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1543851204-41186-15-git-send-email-pbonzini@redhat.com>
Message-Id: <1550165756-21617-10-git-send-email-pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>