python: Fix Param initialization issue in Python 3

python: Fix Param initialization issue in Python 3

When initializing a param with a SimObject NULL pointer, convert()
checks if the 'ptype' attribute has been created and whether the value
is NULL. In that case, it assumes that the object is being
initizalized as a part of SimObject initialization and defers the
conversion. This check is implemented using hasattr() which in turn is
implemented using the __getattr__ implementation that asserts because
all SimObjects haven't been initialized yet.

Implement the check using a lookup in the object's dictionary instead
to prevent the SimObject lookup.

Change-Id: I7367563c4fb71f6d2be541ebdc0be418e9f73d48
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15990
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>