virbuf: fix const-correctness

virbuf: fix const-correctness

Although the compiler wasn't complaining (since it was the pointer,
rather than what was being pointed to, that was actually const), it
looks quite suspicious to call a function with an argument labeled
const when the nature of the pointer (virBufferPtr) is hidden behind
a typedef.  Dropping const makes the function declarations easier
to read.

* src/util/buf.h: Drop const from all functions that modify buffer
argument.
* src/util/buf.c (virBufferSetError, virBufferAdd)
(virBufferContentAndReset, virBufferFreeAndReset)
(virBufferAsprintf, virBufferVasprintf, virBufferEscapeString)
(virBufferEscapeSexpr, virBufferEscape): Fix fallout.