qcow2: Silence clang -m32 compiler warning

qcow2: Silence clang -m32 compiler warning

With -m32, size_t is generally only a uint32_t.  That makes clang
complain that in the assertion

  assert(qiov->size <= INT64_MAX);

the range of the type of qiov->size (size_t) is too small for any of its
values to ever exceed INT64_MAX.

Cast qiov->size to uint64_t to silence clang.

Fixes: f7ef38dd1310d7d9db76d0aa16899cbc5744f36d
       ("block: use int64_t instead of uint64_t in driver read
       handlers")
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211011155031.149158-1-hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>