curl: convert CURLAIOCB to byte values

This is in preparation for the conversion from bdrv_aio_readv to
bdrv_co_preadv, and it also requires changing some of the size_t values
to uint64_t.  This was broken before for disks > 2TB, but now it would
break at 4GB.

Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170515100059.15795-6-pbonzini@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>