libvhost-user: replace qemu/memfd.h usage

libvhost-user: replace qemu/memfd.h usage

Undo the damage from commit 5f9ff1eff3 ("libvhost-user: Support tracking
inflight I/O in shared memory") which introduced glib dependency through
osdep.h inclusion.

libvhost-user.c tries to stay free from glib usage.

Use glibc memfd_create directly when available (assumed so when
MFD_ALLOW_SEALING is defined). A following commit will make the project
standalone and check for memfd API at configure time, instead of a
panic at runtime.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20201125100640.366523-3-marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>