util: virerror: Avoid a copy of the error messages

util: virerror: Avoid a copy of the error messages

Some error message reporting functions already have allocated buffers
which were used to format the error message, so copying the strings is
redundant.

Extract the internals from 'virRaiseErrorFull' to
'virRaiseErrorInternal' which takes allocated strings as arguments and
steals them, so that callers can reuse the buffers.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>