util: buffer: define cleanup function using VIR_DEFINE_AUTOPTR_FUNC

util: buffer: define cleanup function using VIR_DEFINE_AUTOPTR_FUNC

Using the new VIR_DEFINE_AUTOPTR_FUNC macro defined in
src/util/viralloc.h, define a new wrapper around an existing
cleanup function which will be called when a variable declared
with VIR_AUTOPTR macro goes out of scope. Also, drop the redundant
viralloc.h include, since that has moved from the source module into
the header.

When variables of type virBufferPtr and virBufferEscapePairPtr
are declared using VIR_AUTOPTR, the functions virBufferFreeAndReset
and virBufferEscapePairFree, respectively, will be run automatically
on them when they go out of scope.

Signed-off-by: Sukrit Bhatnagar <skrtbhtngr@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>