virtiofsd: Add a helper to send element on virtqueue

We have open coded logic to take locks and push element on virtqueue at
three places. Add a helper and use it everywhere. Code is easier to read and
less number of lines of code.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Message-Id: <20210930153037.1194279-5-vgoyal@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>