curl: Store BDRVCURLState pointer in CURLSocket

A socket does not really belong to any specific state.  We do not need
to store a pointer to "its" state in it, a pointer to the common
BDRVCURLState is sufficient.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210309130541.37540-2-mreitz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>