Wrap command domfstrim

Signed-off-by: Kyla Zhang <weizhan@redhat.com>
Add config for domfstrim

Signed-off-by: Kyla Zhang <weizhan@redhat.com>
Add test for domfstrim

Signed-off-by: Kyla Zhang <weizhan@redhat.com>
libvirt-tests: virsh_domfstrim, remove global using.

Not necessary to use global variable, just adding a new get_diskmap_size()
call outside can make it work well.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>