hw/9pfs: Introduce tracing for 9p pdu handlers

Plan is to replace the existing debug infrastructure with Qemu tracing
infrastructure so that user can dynamically enable/disable trace events and
therefore a meaningful trace log can be generated which can be further
filtered using an analysis script.

Note: Because of current simpletrace limitations, the trace events are
logging at max 6 args, however, once the more args are supported, we can
change trace events to log more info as well. Also, This initial patch only
provides a replacement for existing debug infra. More trace events to be
added later for newly added handlers and sub-routines.

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>