core: allow read-only for VirtIO-SCSI DirectLUN disks

In order to support read-only property for VirtIO-SCSI
DirectLUN disks, the device type for such disks should
be 'disk' (SCSI emulation) instead of 'lun' (SCSI pass-through).
I.e. representation in the domain xml sent to libvirt should
look like [1] instead of [2].

Note that this limitation (enabling pass-through for read-only
VirtIO-SCSI DirectLUN disks) could be removed once 'Support
read only VirtIO-SCSI direct LUNs in QEMU' bug [3] is resolved.

To provide full flexibility, added a new check-box for enabling
pass-through (see screenshot [4]). I.e. the user can choose between
emulated and pass-through ('disk' vs. 'lun'); read-only is
currently supported only for emulated.

[1] <disk type='block' device='disk'>...</disk>
[2] <disk type='block' device='lun'>...</disk>
[3] https://bugzilla.redhat.com/1110396
[4] screenshot: http://i.imgur.com/5mS3xSk.jpg

Change-Id: Ic317722be9ef8978b50352c12bc06ea8c906dcd0
Bug-Url: https://bugzilla.redhat.com/1125373
Signed-off-by: Daniel Erez <derez@redhat.com>