[Fedora-xen] enlarge file backed filesystem

[Jeff]

I have created an xp domain and fc6 domain but I picked too small of 
size for the image.

My xp domain is using a physical device (logical volume) and my fc6 
domain is using a file for the mapped disk.

Isn't there a way to enlarge the filesystem of the guest by enlarging 
the backing file/device?  If so, what exactly do you do to enlarge the 
backing file/dev?

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Thanks,
Jeff