qemuxml2argvtest: Sanitize testing of '-enable-fips'

qemuxml2argvtest: Sanitize testing of '-enable-fips'

Rename 'FLAG_FIPS' to 'FLAG_FIPS_HOST' to signify that we are simulating
a host supporting fips mode and use the flag to assert 'enabeFips'
argument of 'qemuProcessCreatePretendCmdBuild' rather than passing it
via QEMU_CAPS_ENABLE_FIPS.

This prepares the testsuite for testing of -enable-fips deprecation in
qemu-5.2.

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