conf: Check for hostdev conflicts when assign default disk address

conf: Check for hostdev conflicts when assign default disk address

https://bugzilla.redhat.com/show_bug.cgi?id=1210587  (completed)

When generating the default drive address for a SCSI <disk> device,
check the generated address to ensure it doesn't conflict with a SCSI
<hostdev> address. The <disk> address generation algorithm uses the
<target> "dev" name in order to determine which controller and unit
in order to place the device. Since a SCSI <hostdev> device doesn't
require a target device name, its placement on the guest SCSI address
"could" conflict.  For instance, if a SCSI <hostdev> exists at
controller=0 unit=0 and an attempt to hotplug 'sda' into the guest
made, there would be a conflict if the <hostdev> is already using
/dev/sda.