virtiofs: Set FR_SENT flag only after request has been sent

FR_SENT flag should be set when request has been sent successfully sent
over virtqueue. This is used by interrupt logic to figure out if interrupt
request should be sent or not.

Also add it to fqp->processing list after sending it successfully.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>