virttest: Add a module for cleaning up enviroment

With this module, tests can define multiple clean up functions which will
be called after tests termination inside the tests scripts. These will help
tests which need complex host enviroment setup and clean up.

The clean up function can be defined in tests and will be register to a
global list named with exithandlers of the test. You can register the
clean up functions right after the set up operation finished. And they
will be executed no matter the test is failed or not after the test finished.
And the execution order will be the backward of the register order.

Changes from v1:
 * Fixed typo, removed unused module and trailing whitespaces.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
qemu.tests: Use funcatexit to handle the clean up in transparent hugepage test

Use funcatexit to handle the debugfs clean up in transparent hugepage test

Signed-off-by: Yiqiao Pu <ypu@redhat.com>