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fix(virtio/fs): do not double num_request_queues in Fs::num_queues

Fs::num_queues() multiplied config.num_request_queues by 2 when
computing the total virtqueue count. Unlike virtio-net queue pairs,
each virtio-fs request queue is a single virtqueue (VirtIO 1.4 Sec
5.11.2 and 5.11.4): queue 0 is hiprio, followed by an optional
notification queue when VIRTIO_FS_F_NOTIFICATION is offered, followed
by num_request_queues request queues.

Doubling num_request_queues exposed extra phantom queues to the guest
and caused VuFs::new() to compute the wrong num_request_queues when
paired with an Alioth vhost-user fs backend.

Add num_request_queues directly without multiplying by 2.

Fixes: 56c99f36b324 ("feat(fs): add a VirtIO FS device skeleton")
Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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