qemurunner: more cleanups for output blocking

qemurunner: more cleanups for output blocking

If we are only tracking stdout and are not using self.readsock we end
up throwing an exception blocking further action from the thread. Fix
this by checking self.readsock is not None first.

While we are at it split even into fd, event to make things clearer
and handle the fail path of stringify_event by echoing the hex value
of the unknown flag.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Mikko Rapeli <mikko.rapeli@linaro.org>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>