qemu: Fix hot unplug of SCSI_HOST device

qemu: Fix hot unplug of SCSI_HOST device

https://bugzilla.redhat.com/show_bug.cgi?id=1141732

Introduced by commit id '8f76ad99' the logic to detach a scsi_host
device (SCSI or iSCSI) fails when attempting to remove the 'drive'
because as I found in my investigation - the DelDevice takes care of
that for us.

The investigation turned up commits to adjust the logic for the
qemuMonitorDelDevice and qemuMonitorDriveDel processing for interfaces
(commit id '81f76598'), disk bus=VIRTIO,SCSI,USB (commit id '0635785b'),
and chr devices (commit id '55b21f9b'), but nothing with the host devices.

This commit uses the model for the previous set of changes and applies
it to the hostdev path. The call to qemuDomainDetachHostSCSIDevice will
return to qemuDomainDetachThisHostDevice handling either the audit of
the failure or the wait for the removal and then call into
qemuDomainRemoveHostDevice for the event, removal from the domain hostdev
list, and audit of the removal similar to other paths.

NOTE: For now the 'conn' param to +qemuDomainDetachHostSCSIDevice is left
as ATTRIBUTE_UNUSED.  Removing requires a cascade of other changes to be
left for a future patch.