libqtest: add qtest_remove_abrt_handler()

Add a function to remove previously-added abrt handler functions.

Now that a symmetric pair of add/remove functions exists we can also
balance the SIGABRT handler installation. The signal handler was
installed each time qtest_add_abrt_handler() was called. Now it is
installed when the abrt handler list becomes non-empty and removed again
when the list becomes empty.

The qtest_remove_abrt_handler() function will be used by
vhost-user-blk-test.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210223144653.811468-5-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>