kvm: fix bug while fetching -device list

_GetKVMOutput expects the command to succeed, but unfortunately on some
versions of kvm "-device ?" will output a correct list of devices, while
exiting with an error code.

To fix this we accept failure in that case (note that this doesn't
change the situation when this command is not supported, as in that case
the output will still not contain the correct string).

This patch also updates the docstring, adding the parameter types and
description.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>