KEXEC: disconnect all PCI devices from the PCI bus on crash

KEXEC: disconnect all PCI devices from the PCI bus on crash

In the case of a crash, IOMMU DMA remapping gets turned off so that
the kdump kernel may boot.  However, this is warned as being dangerous
in the VTD specification if a DMA transaction is in progress.

Also, in the case of a crash, DMA transactions and interrupts from
peripheral devices such as network cards are likely to keep coming in.
Without DMA remapping enabled, the transactions will be writing over
low memory, corrupting the crash state, and perhaps even the kdump
reserved memory.

Therefore, on the crash path, we can disconnect all PCI devices from
their respective buses so that they are no longer able to be DMA
busmasters.  This reduces the risk of DMA transactions corrupting
state (and will also reduce spurious interrupts arriving to the kdump
kernel) until the kdump kernel and properly reset the PCI devices.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>