When unregister memory listeners, we should call, e.g.,
region_del() (and possibly other undo operations) on every existing
memory region sections there, otherwise we may leak resources that are
held during the region_add(). This patch undo the stuff for the
listeners, which emulates the case when the address space is set from
current to an empty state.

I found this problem when debugging a refcount leak issue that leads to
a device unplug event lost (please see the "Bug:" line below).  In that
case, the leakage of resource is the PCI BAR memory region refcount.
And since memory regions are not keeping their own refcount but onto
their owners, so the vfio-pci device's (who is the owner of the PCI BAR
memory regions) refcount is leaked, and event missing.

We had encountered similar issues before and fixed in other
way (ee4c1128, "vhost: Release memory references on cleanup"). This
patch can be seen as a more high-level fix of similar problems that are
caused by the resource leaks from memory listeners. So now we can remove
the explicit unref of memory regions since that'll be done altogether
during unregistering of listeners now.

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=1531393


Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180122060244.29368-5-peterx@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>