Add AHCI support to qemu driver

Add AHCI support to qemu driver

Tested with multiple AHCI controllers and multiple disks attached
to a controller. E.g.,

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/test/disk0.raw'/>
      <target dev='sda' bus='sata'/>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/test/disk1.raw'/>
      <target dev='sdb' bus='sata'/>
      <address type='drive' controller='0' bus='0' unit='1'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/test/disk2.raw'/>
      <target dev='sdc' bus='sata'/>
      <address type='drive' controller='1' bus='0' unit='0'/>
    </disk>
    <controller type='sata' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </controller>
    <controller type='sata' index='1'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </controller>