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

[Richard W.M. Jones]

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.