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

[Richard W.M. Jones]

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.