util: error: define cleanup function using VIR_DEFINE_AUTOPTR_FUNC

util: error: 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 a variable of type virErrorPtr is declared using
VIR_AUTOPTR, the function virFreeError will be run
automatically on it when it goes out of scope.

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