[libvirt] [PATCH v3] Add macro for handling exponential backoff loops.

v2 was here: https://www.redhat.com/archives/libvir-list/2016-April/thread.html#00355 v2->v3: - Rename virTimeBackOffInit -> Start. - virTimeBackOffStart now returns an error indication. - Remove var->first and var->timeout fields from struct. - Change var->limit field name to limit_t. - Remove '- 2' fudge factor. - virTimeBackOffCondition returns bool instead of int. I didn't remove the macro hiding the 'while' keyword because I prefer the current user interface. I retested this against current libguestfs. Rich.