xen-platform: unplug AHCI disks

xen-platform: unplug AHCI disks

To support Xen guests using the Q35 chipset, the unplug protocol needs
to also remove AHCI disks.

Make pci_xen_ide_unplug() more generic, iterating over the children
of the PCI device and destroying the "ide-hd" devices. That works the
same for both AHCI and IDE, as does the detection of the primary disk
as unit 0 on the bus named "ide.0".

Then pci_xen_ide_unplug() can be used for both AHCI and IDE devices.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>