qemu: Label master key file

qemu: Label master key file

When creating the master key, we used mode 0600 (which we should) but
because we were creating it as root, the file is not readable by any
qemu running as non-root.  Fortunately, it's just a matter of labelling
the file.  We are generating the file path few times already, so let's
label it in the same function that has access to the path already.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>