xenapi: Don't swallow missing SR exception

The existing code would catch a missing SR exception and bail out of the
`update_status` method before finishing, causing required keys to be missing.
The end-result was code breaking later on when it tried to access these keys.

Since an SR is required to generate these required stats keys, we shouldn't
swallow the StorageRepositoryNotFound exception and instead should fail-fast.

Fixes bug 1175357

Change-Id: I3d65ae807fb2d86277bc0061c1967a87792b6662