virConnectGetCapabilities (second version) (was: Re: [Libvir] [PATCH] Add virConnectGetCapabilities call to return the capabilities of the driver / hypervisor)

This is the second version of this patch. I'll just summarise what's changed below rather than reiterating what it does over again. I believe this addresses everyone's concerns, _except_ for a rather long email from Dan to which I'm going to provide a separate reply. Changes over previous patch: * strjoin is gone, and replaced by virBuffer* [DV] ** however, in order to do that I had to duplicate the buffer handling code for qemud (since qemud doesn't link with libvirt and the virBuffer* code depends on other internals of libvirt). * <bits> is now optional, and where it's too hard to determine it reliably, we don't include it in the XML output [DV, danpb] * <domain><type> -> <domain_type> [markmc] Rich.