'+' in strings get translated to ' ' when editing domains.
While xenDaemonDomainCreateXML() did URL-escape the sexpr,
xenDaemonDomainDefineXML() did not.

Remove the explicit urlencode() in xenDaemonDomainCreateXML() and add
the direct encoding calls to xend_op_ext() because it calls xend_post()
which uses "Content-Type: application/x-www-form-urlencoded". According
to <http://www.w3.org/TR/html4/interact/forms.html#h-17.13.4.1> this
requires all parameters to be url-encoded as specified in rfc1738.

Notice: virBufferAsprintf(..., "%s=%s", ...) is again replaced by three
calls to virBufferURIEncodeString() and virBufferAddChar() because '='
is a "reserved" character, which would get escaped by
virBufferURIEncodeString(), which - by the way - escapes anything not
c_isalnum().
Signed-off-by: NPhilipp Hahn <hahn@univention.de>