ata_piix: defer disks to the Hyper-V drivers by default

When we are hosted on a Microsoft Hyper-V hypervisor the guest disks
are exposed both via the Hyper-V paravirtualised drivers and via an
emulated SATA disk drive.  In this case we want to use the paravirtualised
drivers if we can as they are much more efficient.  Note that the Hyper-V
paravirtualised drivers only expose the virtual hard disk devices, the
CDROM/DVD devices must still be enumerated.

Mark the host controller ATA_HOST_IGNORE_ATA to prevent enumeration of
disk devices.

BugLink: http://bugs.launchpad.net/bugs/929545
BugLink: http://bugs.launchpad.net/bugs/942316


Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>