nbd: Clean up ioctl handling of qemu-nbd -c

The kernel ioctl() interface into NBD is limited to 'unsigned long';
we MUST pass in input with that type (and not int or size_t, as
there may be platform ABIs where the wrong types promote incorrectly
through var-args).  Furthermore, on 32-bit platforms, the kernel
is limited to a maximum export size of 2T (our BLKSIZE of 512 times
a SIZE_BLOCKS constrained by 32 bit unsigned long).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1463006384-7734-8-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
