virsh.capabilities: Check power management

1) Add checking for power management support again result of
   `pm-is-supported`
2) Rewrite all XML parsing using libvirt_xml module.

Signed-off-by: Hao Liu <hliu@redhat.com>
add test for virt-xml-validate

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

virt_xml_validate: fix an spelling error in cfg and provide an interface for other schemas.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Add test for virt-top.

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

virt_top: remove an unused variable.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Merge pull request #261 from yangdongsheng/virt_cmd_lxc

Add test for virt-top and virt-xml-validate. (qemu & lxc)
Merge pull request #247 from Hao-Liu/check_capabilities_pm

virsh.capabilities: Check power management