qtest: add a QOM object for qtest

The qtest server right now can only be created using the -qtest
and -qtest-log options.  Allow an alternative way to create it
using "-object qtest,chardev=...,log=...".

This is part of the long term plan to make more (or all) of
QEMU configurable through QMP and preconfig mode.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>