libvirt: handle exception while get vcpu info

If an exception is raised while get a libvirt domain's vcpu info,
the update_available_resource periodic task will be failed, which
will result in the resource of this compute node will never be
reported.

This patch add an exception handling to avoid this situation.
Closes-bug: #1270008

Change-Id: I69109402416989fdaa421f8dbc72953bd067c407
Merge "libvirt: handle exception while get vcpu info"