passthrough: support passthrough in stubdoms

passthrough: support passthrough in stubdoms

Few changes are needed in qemu to enable pci passthrough in stubdoms:

- we need to explicitely ask for PCI_FILL_IDENT and PCI_FILL_CLASS to
pcilib, otherwise it won't return them by default;

- we need to postpone pcilib initialization otherwise it will fail
because the pci backend is not configured yet in the pci hotplug case;

- we have to read the pci related nodes on xenstore from the guest's
path.

[PATCH 7 of 7] qemu: passthrough support in stubdoms
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>