configure, meson: allow enabling vhost-user on all POSIX systems

With the possibility of using a pipe pair via qemu_pipe() as a
replacement on operating systems that doesn't support eventfd,
vhost-user can also work on all POSIX systems.

This change allows enabling vhost-user on all non-Windows platforms
and makes libvhost_user (which still depends on eventfd) a linux-only
feature.

Signed-off-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20220304100854.14829-4-slp@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>