vm_xml: Update seclabel functions to support multiple seclabels

Current functions only support one seclabel in domain xml, while
domain xml support at least two seclabels in domain xml, depend on
security drivers, our testing is focus on selinux and DAC.

e.g.
...
  <seclabel type='none' model='dac'/>
  <seclabel type='none' model='selinux'/>
...

so, modify set_seclabel and get_seclabel functions use list contains
dict member as parameter and return.

Signed-off-by: Wayne Sun <gsun@redhat.com>
libvirt_xml_unittest: Update unit test for seclabel

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #1794 from waynesun09/seclabel_xml

vm_xml: Update seclabel functions to support multiple seclabels