domainMonitor: Fix unsafe iteration

Using dict.iteritems() is safe only if nobody else can modify the
dictionary while iterating. However in DomainMonitor, others threads can
add or remove items during the iteration, so we must use dict.items(),
which copies the keys and values atomically.

dict.iteritems() is useful when we have huge dict and we don't want to
create a huge list of items, but in case of the domain monitor, where we
have less then 100 monitors, there is not need for fancy iteration.

While modifying this line, I also improve the names to make the code
more clear, this can be separated to another patch but I don't think it
is needed in this case.

Change-Id: I5313d25c7148e4a0362d19fe31d2e78f94c26e39
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29007
Reviewed-by: Yoav Kleinberger <ykleinbe@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>