conf: use disk source accessors in security/

conf: use disk source accessors in security/

Part of a series of cleanups to use new accessor methods.

* src/security/security_dac.c (virSecurityDACSetSecurityImageLabel)
(virSecurityDACRestoreSecurityImageLabelInt)
(virSecurityDACSetSecurityAllLabel): Use accessors.
* src/security/security_selinux.c
(virSecuritySELinuxRestoreSecurityImageLabelInt)
(virSecuritySELinuxSetSecurityImageLabel)
(virSecuritySELinuxSetSecurityAllLabel): Likewise.

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