ahci: properly reset PxCMD on HBA reset

While testing q35, I found that windows 7 (specifically, windows 7 ultimate
with sp1 x64), wouldn't install because it can't find the cdrom or disk drive.
The failure message is: 'A required cd/dvd device driver is missing. If you
have a driver floppy disk, CD, DVD, or USB flash drive, please insert it now.'
This can also be reproduced on piix by adding an ahci controller, and
observing that windows 7 does not see any devices behind it.

The problem is that when windows issues a HBA reset, qemu does not reset the
individual ports' PxCMD register. Windows 7 then reads back the PxCMD register
and presumably assumes that the ahci controller has already been initialized.
Windows then never sets up the PxIE register to enable interrupts, and thus it
never gets irqs back when it sends ata device inquiry commands.

This change brings qemu into ahci 1.3 specification compliance.

Section 10.4.3 HBA Reset:

"
When GHC.HR is set to '1', GHC.AE, GHC.IE, the IS register, and all port
register fields (except PxFB/PxFBU/PxCLB/PxCLBU) that are not HwInit in the
HBA's register memory space are reset.
"

I've also re-tested Fedora 16 and 17 to verify that they continue to work with
this change.

Signed-off-by: Jason Baron <jbaron@redhat.com>
Acked-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
