dom0 state dump

dom0 state dump

Since xenctx cannot (for obvious reasons) display the context of
dom0's vCPU-s, here are the beginnings of a console based mechanism to
achieve the same (useful if dom0 hangs with one or more de-scheduled
vCPU-s). The stack handling obviously needs improvement, but the
register context should come out fine in all cases.

Signed-off-by: Jan Beulich <jbeulich@novell.com>