xen/x86: hap: Clean-up and harden hap_enable()

xen/x86: hap: Clean-up and harden hap_enable()

Unlike shadow_enable(), hap_enable() can only be called once during
domain creation and with the mode equal to
PG_external | PG_translate | PG_refcounts.

If it were called twice, then we might have some interesting problems
as the p2m tables would be re-allocated (and therefore all the mappings
would be lost).

Add code to sanity check the mode and that the function is only called
once. Take the opportunity to an if checking that PG_translate is set.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Jan Beulich <jbeulich@suse.com>