qemu-img: clarify src_cache option documentation

The source cache option takes the same values as the cache option.  The
documentation reads a little strange because it starts with "In contrast
the src_cache option ...".  The fact that this is comparing with the
previous documented option (the 'cache' option) is implicit.  Readers
may be confused, especially if they jump to src_cache without reading
cache documentation first.

Suggested-by: Jeff Nelson <jenelson@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
qemu-img: fix rebase src_cache option documentation

The src_cache option (-T) specifies the cache mode for backing files.
It applies both the image's old backing file as well as the new backing
file:

  ret = bdrv_open(&bs_old_backing, backing_name, NULL, NULL, src_flags,
                  old_backing_drv, &local_err);
  if (ret) {
      ...
  }
  if (out_baseimg[0]) {
      bs_new_backing = bdrv_new("new_backing", &error_abort);
      ret = bdrv_open(&bs_new_backing, out_baseimg, NULL, NULL, src_flags,
                      new_backing_drv, &local_err);
      if (ret) {
          ...
      }
  }

The documentation only mentions the new backing file but it really
applies to both.

Suggested-by: Jeff Nelson <jenelson@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
block/archipelago: Use QEMU atomic builtins

Replace __sync builtins with ones provided by QEMU
for atomic operations.

Special thanks goes to Paolo Bonzini for his refactoring
suggestion in order to use the already existing atomic builtins
interface.

Signed-off-by: Chrysostomos Nanakos <cnanakos@grnet.gr>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>