sockets: fix parsing of ipv4/ipv6 opts in parse_socket_addr

The inet_parse() function looks for 'ipv4' and 'ipv6' flags, but only
treats them as bare bool flags. The normal QemuOpts parsing would allow
on/off values to be set too.

This updates inet_parse() so that its handling of the 'ipv4' and 'ipv6'
flags matches that done by QemuOpts.

This impacts the NBD block driver parsing the legacy filename syntax and
the migration code parsing the socket scheme.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20180125171412.21627-1-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>