sockets: Make abstract UnixSocketAddress depend on CONFIG_LINUX

sockets: Make abstract UnixSocketAddress depend on CONFIG_LINUX

The abstract socket namespace is a non-portable Linux extension.  An
attempt to use it elsewhere should fail with ENOENT (the abstract
address looks like a "" pathname, which does not resolve).  We report
this failure like

    Failed to connect socket abc: No such file or directory

Tolerable, although ENOTSUP would be better.

However, introspection lies: it has @abstract regardless of host
support.  Easy enough to fix: since Linux provides them since 2.2,
'if': 'defined(CONFIG_LINUX)' should do.

The above failure becomes

    Parameter 'backend.data.addr.data.abstract' is unexpected

I consider this an improvement.

Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Signed-off-by: Daniel P. Berrangé <[email protected]>