hw/block/nvme: rename __nvme_zrm_open

hw/block/nvme: rename __nvme_zrm_open

Get rid of the (reserved) double underscore use. Rename the "generic"
zone open function to nvme_zrm_open_flags() and add a generic `int
flags` argument instead which allows more flags to be easily added in
the future. There is at least one TP under standardization that would
add an additional flag.

Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>