hostmem-file: add "align" option

When mmap(2) the backend files, QEMU uses the host page size
(getpagesize(2)) by default as the alignment of mapping address.
However, some backends may require alignments different than the page
size. For example, mmap a device DAX (e.g., /dev/dax0.0) on Linux
kernel 4.13 to an address, which is 4K-aligned but not 2M-aligned,
fails with a kernel message like

[617494.969768] dax dax0.0: qemu-system-x86: dax_mmap: fail, unaligned vma (0x7fa37c579000 - 0x7fa43c579000, 0x1fffff)

Because there is no common approach to get such alignment requirement,
we add the 'align' option to 'memory-backend-file', so that users or
management utils, which have enough knowledge about the backend, can
specify a proper alignment via this option.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Message-Id: <20171211072806.2812-2-haozhong.zhang@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[ehabkost: fixed typo, fixed error_setg() format string]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

docs/nvdimm.txt

@@ -122,3 +122,19 @@ Note:
      M >= size of RAM devices +
           size of statically plugged vNVDIMM devices +
           size of hotplugged vNVDIMM devices
+
+Alignment
+---------
+
+QEMU uses mmap(2) to maps vNVDIMM backends and aligns the mapping
+address to the page size (getpagesize(2)) by default. However, some
+types of backends may require an alignment different than the page
+size. In that case, QEMU v2.12.0 and later provide 'align' option to
+memory-backend-file to allow users to specify the proper alignment.
+
+For example, device dax require the 2 MB alignment, so we can use
+following QEMU command line options to use it (/dev/dax0.0) as the
+backend of vNVDIMM:
+
+ -object memory-backend-file,id=mem1,share=on,mem-path=/dev/dax0.0,size=4G,align=2M
+ -device nvdimm,id=nvdimm1,memdev=mem1