security_dac: Set DAC label on SGX /dev nodes

security_dac: Set DAC label on SGX /dev nodes

As advertised in previous commits, QEMU needs to access
/dev/sgx_vepc and /dev/sgx_provision files when SGX memory
backend is configured. And if it weren't for QEMU's namespaces,
we wouldn't dare to relabel them, because they are system wide
files. But if namespaces are used, then we can set label on
domain's private copies, just like we do for /dev/sev.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Haibin Huang <haibin.huang@intel.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>