tests: use memfd in vhost-user-test

tests: use memfd in vhost-user-test

This will exercise the memfd memory backend and should generally be
better for testing than memory-backend-file (thanks to anonymous files
and sealing).

If memfd is available, it is preferred.

However, in order to check that file & memfd backends both work
correctly, the read-guest-mem test is checked explicitly for each.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180201132757.23063-8-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>