Add missing checks for read only connections

Add missing checks for read only connections

As pointed on CVE-2011-1146, some API forgot to check the read-only
status of the connection for entry point which modify the state
of the system or may lead to a remote execution using user data.
The entry points concerned are:
  - virConnectDomainXMLToNative
  - virNodeDeviceDettach
  - virNodeDeviceReAttach
  - virNodeDeviceReset
  - virDomainRevertToSnapshot
  - virDomainSnapshotDelete

* src/libvirt.c: fix the above set of entry points to error on read-only
                 connections