Change where transmitter packet cache is created

Change where transmitter packet cache is created

The transmitter packet cache suffers from a similar issue as the
receiver packet cache did before commit 0dda5aa8 "Partially revert
commit ab655bb1...". If the VM goes through a suspend/resume cycle
with queued transmits then the cache would be torn down before all
objects had been freed, leading to ASSERTion failures in checked
builds and memory corruption in free builds.

This patch moves creation of the packet cache from the 'connect'
phase to the 'initialization' phase to avoid this problem.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>