hw/block/nvme: refactor nvme_dma

The nvme_dma function doesn't just do DMA (QEMUSGList-based) memory transfers;
it also handles QEMUIOVector copies.

Introduce the NvmeTxDirection enum and rename to nvme_tx. Remove mapping
of PRPs/SGLs from nvme_tx and instead assert that they have been mapped
previously. This allows more fine-grained use in subsequent patches.

Add new (better named) helpers, nvme_{c2h,h2c}, that does both PRP/SGL
mapping and transfer.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>