vat: fix socket connection

vat_socket_connect(...) needs to set the api main client index. The M2
helper macro needed to allocate a buffer of size sizeof(*mp) + n, not
sizeof(*mp).

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I7abf35d2ba045d82765bd27f88899287fd351602