VIR_AUTODISPOSE_STR is similar to VIR_AUTOFREE(char *) but uses
virDispose for clearing of the stored string.

This patch also refactors VIR_DISPOSE to use the new helper which is
used for the new macro.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>