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>
Merge pull request #247 from Hao-Liu/check_capabilities_pm

virsh.capabilities: Check power management
svirt_start_destroy: Add test cases with security_driver as none

The setting in qemu conf affects the behavior when starts domain with
seclabel, by default the security_driver is selinux and dac, in this
commit add the security_driver as 'none' test cases.

Signed-off-by: Wayne Sun <gsun@redhat.com>
svirt_start_destroy: Add bug number to label restore FAIL cases

The label restore problem is a long time existing bug we are aware of,
and no fix in upstream yet, check more info in:
https://bugzilla.redhat.com/show_bug.cgi?id=547546

Signed-off-by: Wayne Sun <gsun@redhat.com>
svirt_start_destroy: Update seclabel functions

Using list as given parameter and return according to change in
vm_xml module.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Update set_seclabel use list in svirt test cases

Using list as given parameter according to vm_xml module change.

Signed-off-by: Wayne Sun <gsun@redhat.com>
svirt_start_destroy: Add cases for multiple seclabels

Add cases for contain both dac and selinux model seclabels, also
with setting in qemu.conf.

Signed-off-by: Wayne Sun <gsun@redhat.com>
svirt_start_destroy: Add test cases for netfs volume

With virt_use_nfs off when selinux enforcing, domain with image on
nfs will fail to start, domain could start while virt_use_nfs on.

Attach netfs disk to domain and set virt_use_nfs, then testing start
domain.

Signed-off-by: Wayne Sun <gsun@redhat.com>