flow: Abstract allocation of new flows with helper function

Currently tcp.c open codes the process of allocating a new flow from the
flow table: twice, in fact, once for guest to host and once for host to
guest connections.  This duplication isn't ideal and will get worse as we
add more protocols to the flow table.  It also makes it harder to
experiment with different ways of handling flow table allocation.

Instead, introduce a function to allocate a new flow: flow_alloc().  In
some cases we currently check if we're able to allocate, but delay the
actual allocation.  We now handle that slightly differently with a
flow_alloc_cancel() function to back out a recent allocation.  We have that
separate from a flow_free() function, because future changes we have in
mind will need to handle this case a little differently.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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