Adds Capabilities()

This adds a new method, Capabilities(), which returns an XML
definition of the hypervisor's system capabilities, much like
`virsh capabilities`.

I've only included integration tests for these two methods due to the
large amount of data returned by the capabilities call.

internal/constants/constants.go

@@ -38,6 +38,7 @@ const (
 const (
 	ProcConnectOpen              = 1
 	ProcConnectClose             = 2
+	ProcConnectGetCapabilties    = 7
 	ProcDomainGetXMLDesc         = 14
 	ProcDomainLookupByName       = 23
 	ProcAuthList                 = 66