Improve exception handling in broker's request processing

When non-fatal exception occurs, try to recover first before
stopping the service mainloop.

Change-Id: I55c5252991b8d428ca64c125b211f974cdaca08b
Signed-off-by: Martin Sivak <msivak@redhat.com>
Save local maintenance flag to shared block in metadata

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1034321
Change-Id: Idaf478c3973bd094a928005c5ea98a4e4fb5159d
Signed-off-by: Martin Sivak <mars@montik.net>
Consider engine to be safe and sound when the state is Running

VDSM should not return this state at all according to it's API
documentation. But it does. Running is returned when VM is Up
and the guest agent is running.

Change-Id: I51507f1ed3c642280b600ee512f4bcc8d5b9bb88
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1034787
Signed-off-by: Martin Sivak <msivak@redhat.com>