There was a duplicated check on debootstrap being installed and only
one has been expanded when cdebootstrap support was added. So
xen-tools bailed out if debootstrap wasn't installed even if it had
been instructed to use cdebootstrap instead.
This also adds support for cdebootstrap-static.
Additionally xt-install-image dispatcher has been extended to hand
array refs of command paths instead of just single command paths.
Thanks to Elmar Heeb for reporting this issue!
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