device-assignment: add configfd_name qdev property

device-assignment: add configfd_name qdev property

When libvirt launches a guest it first chowns the relevenat
/sys/bus/pci/.../config file for an assigned device then drops privileges.

This causes an issue for device assignment because despite being file
owner, the sysfs config space file checks for CAP_SYS_ADMIN before
allowing access to device dependent config space.

This adds a new qdev configfd property which allows libvirt to open the
sysfs config space file and give qemu an already opened file descriptor.
Along with a change pending for the 2.6.35 kernel, this allows the
capability check to compare against privileges from when the file was
opened.

[alex.williamson: make configfd be a string option so that we can pass a
descriptor from libvirt for the hotplug case]

Signed-off-by: Chris Wright <chrisw@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>