qemu: Don't fail if the SCSI host device is shareable between domains

It doesn't make sense to fail if the SCSI host device is specified
as "shareable" explicitly between domains (NB, it works if and only
if the device is specified as "shareable" for *all* domains,
otherwise it fails).

To fix the problem, this patch introduces an array for virSCSIDevice
struct, which records all the names of domain which are using the
device (note that the recorded domains must specify the device as
shareable).  And the change on the data struct brings on many
subsequent changes in the code.

Prior to this patch, the "shareable" tag didn't work as expected,
it actually work like "non-shareable".  So this patch also added notes
in formatdomain.html to declare the fact.

* src/util/virscsi.h:
  - Remove virSCSIDeviceGetUsedBy
  - Change definition of virSCSIDeviceGetUsedBy and virSCSIDeviceListDel
  - Add virSCSIDeviceIsAvailable

* src/util/virscsi.c:
  - struct virSCSIDevice: Change "used_by" to be an array; Add
    "n_used_by" as the array count
  - virSCSIDeviceGetUsedBy: Removed
  - virSCSIDeviceFree: frees the "used_by" array
  - virSCSIDeviceSetUsedBy: Copy the domain name to avoid potential
    memory corruption
  - virSCSIDeviceIsAvailable: New
  - virSCSIDeviceListDel: Change the logic, for device which is already
    in the list, just remove the corresponding entry in "used_by". And
    since it's only used in one place, we can safely removing the code
    to find out the dev in the list first.
  - Copyright updating

* src/libvirt_private.sys:
  - virSCSIDeviceGetUsedBy: Remove
  - virSCSIDeviceIsAvailable: New

* src/qemu/qemu_hostdev.c:
  - qemuUpdateActiveScsiHostdevs: Check if the device existing before
    adding it to the list;
  - qemuPrepareHostdevSCSIDevices: Error out if the not all domains
    use the device as "shareable"; Also don't try to add the device
    to the activeScsiHostdevs list if it already there; And make
    more sensible error w.r.t the current "shareable" value in
    driver->activeScsiHostdevs.
  - qemuDomainReAttachHostScsiDevices: Change the logic according
    to the changes on helpers.

Signed-off-by: Osier Yang <jyang@redhat.com>

docs/formatdomain.html.in

@@ -2798,6 +2798,11 @@
         between domains (assuming the hypervisor and OS support this).
         Only supported by SCSI host device.
         <span class="since">Since 1.0.6</span>
+        <p>
+          Note: Although <code>shareable</code> was introduced
+          <span class="since">in 1.0.6</span>, it did not work as
+          as expected until <span class="since">1.2.2</span>.
+        </p>
       </dd>
     </dl>