blkback/blktap/netback: Fix CVE-2010-3699

blkback/blktap/netback: Fix CVE-2010-3699

A guest can cause the backend driver to leak a kernel
thread. Such leaked threads hold references to the device, whichmakes
the device impossible to tear down. If shut down, the guest remains a
zombie domain, the xenwatch process hangs, and most xm commands will
stop working.

This patch tries to do the following, for all of netback, blkback,
blktap:
    - identify/extract idempotent teardown operations,
    - add/move the invocation of said teardown operation
      right before we're about to allocate new resources in the
      Connected states.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>