qemu: Automatic SCSI controller creation in SCSI disk hotplug broken

qemu: Automatic SCSI controller creation in SCSI disk hotplug broken

When a SCSI disk is hotplugged to a domain that does not have the required
SCSI controller already defined and loaded the following internal error occurs

error: Failed to attach device from scsi_disk.xml
error: internal error: Could not find scsi controller with index 0 required for device

Commit 0260506c added in method qemuBuildDriveDevStr a lookup of the controller
alias. The internal error occurs because in method qemuDomainAttachSCSIDisk
the automatic creation of the potentially missing SCSI controller occurs after
calling qemuBuildDriveDevStr.

This patch reverses the calling sequence.

Signed-off-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
Reviewed-by: Stefan Zimmermann <stzi@linux.vnet.ibm.com>