Right now, let's just support memfd. We could support other shared
memory types later.

vtest adaption based on following QEMU files:

    qemu/include/qemu/memfd.h
    qemu/util/memfd.c

v2: - memfd --> shm
    - don't call fcntl(fd, F_ADD_SEALS) because it fails under callgrind
      for some reason:

    ==8161== Warning: unimplemented fcntl command: 1033
    vtest_new_shm called fcntl which failed (-22)

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-By: Gert Wollny <gert.wollny@collabora.com>