hostmem-memfd: disable for systems without sealing support

hostmem-memfd: disable for systems without sealing support

If seals are not supported, memfd_create() will fail.
Furthermore, there is no way to disable it in this case because
'.seal' property is not registered.

This issue leads to vhost-user-test failures on RHEL 7.2:

  qemu-system-x86_64: -object memory-backend-memfd,id=mem,size=2M,: \
                      failed to create memfd: Invalid argument

and actually breaks the feature on such systems.

Let's restrict memfd backend to systems with sealing support.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Message-Id: <20190311135850.6537-2-i.maximets@samsung.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>