lib: Grab write lock when modifying list of domains

In some places where virDomainObjListForEach() is called the
passed callback calls virDomainObjListRemoveLocked(). Well, this
is unsafe, because the former only grabs a read lock but the
latter modifies the list.
I've identified the following unsafe calls:

- qemuProcessReconnectAll()
- libxlReconnectDomains()

The rest seem to be safe.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>