Add more contants to constants.go
Add more contants to constants.go

* virDomainRunningReason
* virDomainPausedReason
* virDomainXMLFlags

Copied from libvirt.h v1.2.2

libvirt/libvirt-go,rgbkrk/libvirt-go,rgbkrk/libvirt-go,Pryz/libvirt-go,Pryz/libvirt-go,libvirt/libvirt-go,alexzorin/libvirt-go,libvirt/libvirt-go,rgbkrk/libvirt-go,Pryz/libvirt-go,libvirt/libvirt-go,alexzorin/libvirt-go