scripts: Test script to look for -device crashes

Test code to check if we can crash QEMU using -device. It will
test all accel/machine/device combinations by default, which may
take a few hours (it's more than 90k test cases). There's a "-r"
option that makes it test a random sample of combinations.

The scripts contains a whitelist for: 1) known error messages
that make QEMU exit cleanly; 2) known QEMU crashes.

This is the behavior when the script finds a failure:

* Known clean (exitcode=1) errors generate DEBUG messages
  (hidden by default)
* Unknown clean (exitcode=1) errors will generate INFO messages
  (visible by default)
* Known crashes generate error messages, but are not fatal
  (unless --strict mode is used)
* Unknown crashes generate fatal error messages

Having an updated whitelist of known clean errors is useful to make the
script less verbose and run faster when in --quick mode, but the
whitelist doesn't need to be always up to date.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170526181200.17227-4-ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
