x86/hvm: Fix the use of "hap=0" following c/s c0902a9a143a

x86/hvm: Fix the use of "hap=0" following c/s c0902a9a143a

c/s c0902a9a143a refactored hvm_enable() a little, but dropped the logic which
cleared hap_supported in the case that the user had asked for it off.

This results in Xen booting up, claiming:

  (XEN) HVM: Hardware Assisted Paging (HAP) detected but disabled

but with HAP advertised via sysctl, and XEN_DOMCTL_CDF_hap being accepted in
domain_create().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wl@xen.org>
Release-acked-by: Juergen Gross <jgross@suse.com>