block/nvme: Fix use of write-only doorbells page on Aarch64 arch

block/nvme: Fix use of write-only doorbells page on Aarch64 arch

qemu_vfio_pci_map_bar() calls mmap(), and mmap(2) states:

  'offset' must be a multiple of the page size as returned
   by sysconf(_SC_PAGE_SIZE).

In commit f68453237b9 we started to use an offset of 4K which
broke this contract on Aarch64 arch.

Fix by mapping at offset 0, and and accessing doorbells at offset=4K.

Fixes: f68453237b9 ("block/nvme: Map doorbells pages write-only")
Reported-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201029093306.1063879-24-philmd@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>