Description of problem:
libvirt support cache mode "directsync" and "unsafe", but virt-manager has no these two options.

Version-Release number of selected component (if applicable):
virt-manager-0.9.0-14.el6.x86_64
qemu-kvm-0.12.1.2-2.297.el6.x86_64
libvirt-0.9.10-21.el6_3.3.x86_64


How reproducible:
Always.

Steps to Reproduce:
1.Launch virt-manager
2.Open one guest-> show VM hardware details-> click "Add Hardware" button-> choose Storage-> check cache mode option
3.display option: default,none,writeback,writethrough


Actually results:
As above.

Expected results:
cache mode should contain: default,none,writeback,writethrough,directsync,unsafe.

Additional info:
1. refer to man of virsh attach-disk:
# man virsh
...
       attach-disk domain-id source target [--driver driver] [--subdriver subdriver]
       [--cache cache] [--type type] [--mode mode] [--persistent] [--sourcetype
       soucetype] [--serial serial] [--shareable] [--rawio] [--address address]
       [--multifunction]
...
           cache can be one of "default", "none", "writethrough", "writeback", "directsync" or
           "unsafe". 
...

2. results of libvirt-0.9.13-3.el6 of cache mode:
# virsh attach-disk guest /var/lib/libvirt/images/disk5.img vdf --cache unsafe
# virsh dumpxml guest
...
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='unsafe'/>
      <source dev='/var/lib/libvirt/images/disk5.img'/>
      <target dev='vdf' bus='virtio'/>
    </disk>
...

# virsh attach-disk guest /var/lib/libvirt/images/disk6.img vdg --cache directsync
error: Failed to attach disk
error: unsupported configuration: disk cache mode 'directsync' is not supported by this QEMU

# virsh dumpxml guest
...
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='directsync'/>
      <source dev='/var/lib/libvirt/images/disk6.img'/>
      <target dev='vdg' bus='virtio'/>
    </disk>
...