trace: dynamically allocate trace_dstate in CPUState

The CPUState struct has a bitmap tracking which VCPU
events are currently active. This is indexed based on
the event ID values, and sized according the maximum
TraceEventVCPUID enum value.

When we start dynamically assigning IDs at runtime,
we can't statically declare a bitmap without making
an assumption about the max event count. This problem
can be solved by dynamically allocating the per-CPU
dstate bitmap.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1475588159-30598-15-git-send-email-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>