os: deprecate the -enable-fips option and QEMU's FIPS enforcement

os: deprecate the -enable-fips option and QEMU's FIPS enforcement

The -enable-fips option was added a long time ago to prevent the use of
single DES when VNC when FIPS mode is enabled. It should never have been
added, because apps are supposed to unconditionally honour FIPS mode
based on the '/proc/sys/crypto/fips_enabled' file contents.

In addition there is more to achieving FIPS compliance than merely
blocking use of certain algorithms. Those algorithms which are used
need to perform self-tests at runtime.

QEMU's built-in cryptography provider has no support for self-tests,
and neither does the nettle library.

If QEMU is required to be used in a FIPS enabled host, then it must be
built with the libgcrypt library enabled, which will unconditionally
enforce FIPS compliance in any algorithm usage.

Thus there is no need to keep either the -enable-fips option in QEMU, or
QEMU's internal FIPS checking methods.

Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>