Revert c/s 23666:b96f8bdcaa15 KEXEC: disconnect all PCI devices from the PCI bus...

Revert c/s 23666:b96f8bdcaa15 KEXEC: disconnect all PCI devices from the PCI bus on crash

It turns out that this causes all mannor of problems on certain
motherboards (so far with no pattern I can discern)

Problems include:
* Hanging forever checking hlt instruction.
* Panics when trying to change switch root device
* Drivers hanging when trying to check for interrupts.

From: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Keir Fraser <keir@xen.org>
Committed-by: Keir Fraser <keir@xen.org>