virterror: avoid API breakage with vmware

virterror: avoid API breakage with vmware

Fix glitch in commit cddd2a06 (thankfully post-0.8.6, so no
released version has the glitch).

Document and try to workaround glitch in commit 46e9b0f (in 0.8.0),
which invalidated 6 virErrorNumber values dating as far back as 0.7.1.

My audit did not find any other glitches until pre-0.1.0 days.  I'm
not sure how to add a syntax-check off the top of my head, but
hopefully the explicit numbering will make people think twice about
renumbering in the future.

* include/libvirt/virterror.h (virErrorDomain): Avoid inserting
new values in the middle, and add explicit numbering to help avoid
this in the future.
(virErrorNumber): Add explicit numbering, and document the snafu.
* src/remote/remote_driver.c (remoteIO): Compensate for the snafu.