qemu: stop passing -enable-fips to QEMU >= 5.2.0

qemu: stop passing -enable-fips to QEMU >= 5.2.0

Use of the -enable-fips option is being deprecated in QEMU >= 5.2.0. If
FIPS compliance is required, QEMU must be built with libcrypt which will
unconditionally enforce it.

Thus there is no need for libvirt to pass -enable-fips to modern QEMU.
Unfortunately there was never any way to probe for -enable-fips in the
first instance, it was enabled by libvirt based on version number
originally, and then later unconditionally enabled when libvirt dropped
support for older QEMU. Similarly we now use a version number check to
decide when to stop passing -enable-fips.

Note that the qemu-5.2 capabilities are currently from the pre-release
version and will be updated once qemu-5.2 is released.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>