VDI.introduce: raise an exception if the `location` field already exists
VDI.introduce: raise an exception if the `location` field already exists

Previously this happened in the SMAPIv1 plugins, but the SMAPIv3 doesn't
have an explicit notion of `introduce`, instead it just has `Volume.stat`
which will succeed if the `location` is valid.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

salvocambria/xen-api,salvocambria/xen-api,Frezzle/xen-api,simonjbeaumont/xen-api,guard163/xen-api,srowe/xen-api,rafalmiel/xen-api,agimofcarmen/xen-api,jjd27/xen-api,simonjbeaumont/xen-api,thomassa/xen-api,Frezzle/xen-api,guard163/xen-api,djs55/xen-api,salvocambria/xen-api,robertbreker/xen-api,salvocambria/xen-api,djs55/xen-api,djs55/xen-api,euanh/xen-api,euanh/xen-api,thomassa/xen-api,jjd27/xen-api,guard163/xen-api,simonjbeaumont/xen-api,vasilenkomike/xen-api,guard163/xen-api,srowe/xen-api,rafalmiel/xen-api,simonjbeaumont/xen-api,euanh/xen-api,thomassa/xen-api,anoobs/xen-api,vasilenkomike/xen-api,agimofcarmen/xen-api,agimofcarmen/xen-api,thomassa/xen-api,vasilenkomike/xen-api,thomassa/xen-api,jjd27/xen-api,salvocambria/xen-api,djs55/xen-api,euanh/xen-api,djs55/xen-api,anoobs/xen-api,euanh/xen-api,guard163/xen-api,jjd27/xen-api,simonjbeaumont/xen-api,robertbreker/xen-api,guard163/xen-api,vasilenkomike/xen-api,srowe/xen-api,agimofcarmen/xen-api,srowe/xen-api,euanh/xen-api,robertbreker/xen-api,anoobs/xen-api,rafalmiel/xen-api,vasilenkomike/xen-api,Frezzle/xen-api,anoobs/xen-api,robertbreker/xen-api,anoobs/xen-api,thomassa/xen-api,vasilenkomike/xen-api,salvocambria/xen-api,anoobs/xen-api,Frezzle/xen-api,djs55/xen-api,robertbreker/xen-api,rafalmiel/xen-api,agimofcarmen/xen-api,rafalmiel/xen-api,robertbreker/xen-api,srowe/xen-api,Frezzle/xen-api,simonjbeaumont/xen-api,jjd27/xen-api,jjd27/xen-api,agimofcarmen/xen-api,rafalmiel/xen-api