KVM: pci device assignment

Based on a patch from: Amit Shah <amit.shah@qumranet.com>

This patch adds support for handling PCI devices that are assigned to
the guest.

The device to be assigned to the guest is registered in the host kernel
and interrupt delivery is handled.  If a device is already assigned, or
the device driver for it is still loaded on the host, the device
assignment is failed by conveying a -EBUSY reply to the userspace.

Devices that share their interrupt line are not supported at the moment.

By itself, this patch will not make devices work within the guest.
The VT-d extension is required to enable the device to perform DMA.
Another alternative is PVDMA.

Signed-off-by: Amit Shah <amit.shah@qumranet.com>
Signed-off-by: Ben-Ami Yassour <benami@il.ibm.com>
Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>