Catch TypeError exception to avoid setting instance to ERROR state

In libvirt driver, if an instance for some reason is missing a disk,
then calculations in get_disk_over_committed_size_total() throws a
TypeError: int() argument must be a string or a number, not 'NoneType'
exception that may cause the instance to end up in ERROR state.

An example of this happening is in test case test_resize_server_revert
as detailed in the below bug report.

Resolves bug 1161529.

Change-Id: I079da4e271469739504fa87e23149d03652d98f0