add xenpci feature
Created by: HRio
Add a xenpci feature needed for xen driver domains that needs PCI devices already in initramfs.
This is for instance needed for a xen storage driver domain hosting its rootfs on a pci passthrough SAS controller.
/etc/mkinitfs/mkinitfs.conf for a storage driver domain (after this change) with rootfs on md+lvm (/boot provided by dom0)
features="ata base ide scsi usb virtio ext4 xenpci lvm raid"