libxl: add function to attach/detach a disk to/from the local VM

libxl: add function to attach/detach a disk to/from the local VM

Useful if you need to read a guest filesystem (e.g. pygrub).

I'm not overly thrilled with the implementation WRT tap interfaces,
particularly WRT to detach. I was unable to find a way to get at the
paramters necessary to call tap_ctl_destroy so I assumed for now it
that is OK to assume that the tap device is going to be wanted for the
actual domain at some point in the immediate future and hence there is
no pressing need to destroy it.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>