NBD: Use qemu_socket functions to open TCP and UNIX sockets

This commit has the side-effect of making the qemu-nbd binary
capable of binding to IPv6 addresses. ("-b ::1", for instance).
block/nbd.c fails to parse IPv6 IP addresses correctly at this
point, but will work over IPv6 when given a hostname. It still
works over IPv4 as before.

We move the qemu-sockets object from the 'common' to the 'block'
list in the Makefile. The common list includes the block list,
so this is effectively a no-op for the rest of the code.

We also add 32-bit 'magic' attributes to nbd_(request|reply) to
facilitate calculating maximum request/response sizes later.

Signed-off-by: Nick Thomas <nick@bytemark.co.uk>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>