configure: Fix build dependencies with vhost-vdpa.

Following the same logic as for vhost-net-user and vhost-kernel,
enable vhost-net if vhost-net-vdpa is enabled and vhost-net is not
explicitly disabled.
See 299e6f19 ("vhost-net: revamp configure logic")

Autoselect VHOST if VHOST_VDPA is set
See 21c6b0c8 ("configure: simplify vhost condition with Kconfig")
See 2becc36a ("meson: infrastructure for building emulators"

Problems can be triggered using;
... --enable-vhost-vdpa --disable-vhost-user --disable-vhost-kernel ...

Fixes: 108a6481 ("vhost-vdpa: introduce vhost-vdpa backend")
Cc: lulu@redhat.com
Cc: pbonzini@redhat.com
Cc: marcandre.lureau@redhat.com
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20200924210023.160679-1-lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>