virsh: update dumpxml to return CmdResult object

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt: adapt calling of virsh.dumpxml to new @return of CmdResult object.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt-bench: Add test for domstate switch in unixbench.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt_vm: Add dump method to VM

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Add test to run dump when unixbench is running in guest.

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