[libvirt] [PATCH v2] Add macro for handling exponential backoff loops.

Since v1: - Rename the macro VIR_TIME_WHILE_WITH_BACKOFF. - Split out the variable initialization into a separate function that the user must call explicitly. - The macro is now an atomic C statement. An interesting observation about the qemu monitor socket: In some cases it appears almost instantaneously. In other runs it appears after 32-64ms. This latter case is what I expect because qemu startup takes ~50ms on my laptop. There seems to be no reason that I can discern for the difference between the runs. I've not observed the socket appearing anywhere in the middle, only at these two distinct time delays. Rich.