qemu: hot-plug: Fix broken SCSI disk hot-plug

qemu: hot-plug: Fix broken SCSI disk hot-plug

The commit "qemu: hot-plug: Assume support for -device in
qemuDomainAttachSCSIDisk" dropped the code for the automatic SCSI
controller creation used in SCSI disk hot-plugging. If we are
hot-plugging a SCSI disk to a domain and there is no proper SCSI
controller defined, it results in an "error: internal error: Could not
find scsi controller with index X required for device" error.

For that reason reverting a hunk of the commit
d4d32005d6e8b2cc0a2f26b483ca1de10171db6d.

This patch also adds an extra comment to the code to clarify the
loop.

Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.vnet.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>