capabilities: add baselabel per sec driver/virt type to secmodel

capabilities: add baselabel per sec driver/virt type to secmodel

Expand the "secmodel" XML fragment of "host" with a sequence of
baselabel's which describe the default security context used by
libvirt with a specific security model and virtualization type:

<secmodel>
  <model>selinux</model>
  <doi>0</doi>
  <baselabel type='kvm'>system_u:system_r:svirt_t:s0</baselabel>
  <baselabel type='qemu'>system_u:system_r:svirt_tcg_t:s0</baselabel>
</secmodel>
<secmodel>
  <model>dac</model>
  <doi>0</doi>
  <baselabel type='kvm'>107:107</baselabel>
  <baselabel type='qemu'>107:107</baselabel>
</secmodel>

"baselabel" is driver-specific information, e.g. in the DAC security
model, it indicates USER_ID:GROUP_ID.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>