virsh: make negative values with vol-resize more convenient

virsh: make negative values with vol-resize more convenient

When shrinking a volume by a certain size, instead of typing
  vol-resize volume 1G --delta --shrink
we allow the convience of specifying a negative value:
  vol-resize volume -1G --delta --shrink
getting the same results with one more character.

A negative value only makes sense as a delta. Imply the
--delta parameter if the value is negative.

Still require --shrink, because the operation is potentially
destructive.