xen, blkfront: factor out flush-related checks from do_blkif_request()

This change will turn out to be useful when the new multi-queue API
functions will be introduced in the next commit.
xen, blkfront: adapt frontend to multi-queue API with one dummy hctx

This commit adapts the xen-blkfront driver to the multi-queue block
layer API. The interface change is performed so that the driver can
still be compatible with the request API; also, the change is still
restricted to the frontend only, so this commit allows for only one
hardware context, which corresponds to the only available ring.