api: storage: Add the missing write operation support

API_dev_write(va_list ap) is currently lacking the write support
to storage devices because, historically, those devices did not
implement block_write()

The solution has been tested by loading and booting a (patched)
GRUB instance in a QEMU vexpress-a9 environment. The disk write
operations were triggered with GRUB's save_env command.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>