hw/block/nvme: Separate read and write handlers

hw/block/nvme: Separate read and write handlers

The majority of code in nvme_rw() is becoming read- or write-specific.
Move these parts to two separate handlers, nvme_read() and nvme_write()
to make the code more readable and to remove multiple is_write checks
that has been present in the i/o path.

This is a refactoring patch, no change in functionality.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Niklas Cassel <Niklas.Cassel@wdc.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>