Add serial number configuration for drives

[citrus-it]

It's bothered me for a while that running OmniOS under KVM with more than one disk results in only one of the disks showing up in the output of diskinfo.

After a bit of traipse through user-land libraries and into the kernel, it turns out that a virtio drive is given a device ID derived from its serial number, and that libdiskmgmt uses the device ID to determine whether it has already seen a drive (in order to ignore slices for one thing).

Without setting a serial number for a virtio drive, all disks have the same devid and therefore only one shows up.

This patch adds the 'serial' attribute for a drive and, if it is not set, uses the drive index as the serial number. This is sufficient to ensure that each drive gets a unique device ID in the kernel.

With the result:

r151024% pfexec diskinfo
TYPE    DISK                    VID      PID              SIZE          RMV SSD
-       c2t0d0                  Virtio   Block Device      300.00 GiB   no  no
-       c4t0d0                  Virtio   Block Device        8.00 GiB   no  no

r151024% iostat -En
c2t0d0           Soft Errors: 0 Hard Errors: 0 Transport Errors: 0
Vendor: Virtio Product: Block Device Revision: 0000 Serial No: ETC0M3H0M3
Size: 322.12GB <322122547200 bytes>
Media Error: 0 Device Not Ready: 0 No Device: 0 Recoverable: 0
Illegal Request: 0 Predictive Failure Analysis: 0
c4t0d0           Soft Errors: 0 Hard Errors: 0 Transport Errors: 0
Vendor: Virtio Product: Block Device Revision: 0000 Serial No: HONALULU
Size: 8.59GB <8589934592 bytes>