xen-platform: add missing disk unplug option

xen-platform: add missing disk unplug option

The Xen HVM unplug protocol [1] specifies a mechanism to allow guests to
request unplug of 'aux' disks (which is stated to mean all IDE disks,
except the primary master). This patch adds support for that unplug request.

NOTE: The semantics of what happens if unplug of all disks and 'aux' disks
      is simultaneously requests is not clear. The patch makes that
      assumption that an 'all' request overrides an 'aux' request.

[1] http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/misc/hvm-emulated-unplug.markdown

Signed-off-by: Paul Durrant <[email protected]>
Reviewed-by: Stefano Stabellini <[email protected]>
----
Cc: Stefano Stabellini <[email protected]>
Cc: Anthony Perard <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Eduardo Habkost <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: John Snow <[email protected]>