file-win32: Support BDRV_REQ_ZERO_WRITE for truncate

file-win32: Support BDRV_REQ_ZERO_WRITE for truncate

When using bdrv_file, .bdrv_has_zero_init_truncate always returns 1;
therefore, we can behave just like file-posix, and always implement
BDRV_REQ_ZERO_WRITE by ignoring it since the OS gives it to us for
free (note that file-posix.c had to use an 'if' because it shared code
between regular files and block devices, but in file-win32.c,
bdrv_host_device uses a separate .bdrv_file_open).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200428202905.770727-3-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>