xen: rework pci_piix3_xen_ide_unplug

xen: rework pci_piix3_xen_ide_unplug

This is to allow IDE disks to be unplugged when adding to QEMU via:
    -drive file=/root/disk_file,if=none,id=ide-disk0,format=raw
    -device ide-hd,drive=ide-disk0,bus=ide.0,unit=0

as the current code only works for disk added with:
    -drive file=/root/disk_file,if=ide,index=0,media=disk,format=raw

Since the code already have the IDE controller as `dev`, we don't need
to use the legacy DriveInfo to find all the drive we want to unplug.
We can simply use `blk` from the controller, as it kind of was already
assume to be the same, by setting it to NULL.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Message-Id: <20201027154058.495112-1-anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>