Add info about access control checks into API reference

Add info about access control checks into API reference

So that app developers / admins know what access control checks
are performed for each API, this patch extends the API docs
generator to include details of the ACLs for each.

The gendispatch.pl script is extended so that it generates
a simple XML describing ACL rules, eg.

  <aclinfo>
    ...
    <api name='virConnectNumOfDomains'>
      <check object='connect' perm='search_domains'/>
      <filter object='domain' perm='getattr'/>
    </api>
    <api name='virDomainAttachDeviceFlags'>
      <check object='domain' perm='write'/>
      <check object='domain' perm='save' flags='!VIR_DOMAIN_AFFECT_CONFIG|VIR_DOMAIN_AFFECT_LIVE'/>
      <check object='domain' perm='save' flags='VIR_DOMAIN_AFFECT_CONFIG'/>
    </api>
    ...
  </aclinfo>

The newapi.xsl template loads the XML files containing the ACL
rules and generates a short block of HTML for each API describing
the parameter checks and return value filters (if any).

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>