Strip control codes in virBufferEscapeString

Strip control codes in virBufferEscapeString

These cannot be represented in XML.

We have been stripping them, but only if the string had
characters that needed escaping: <>"'&

Extend the strcspn check to include control codes, and strip
them even if we don't do any escaping.

https://bugzilla.redhat.com/show_bug.cgi?id=1184131
https://bugzilla.redhat.com/show_bug.cgi?id=1066564
(cherry picked from commit aeb5262e4397528d582682471cb8075141189465)
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Conflict:
  missing struct testBufAddStrData