x86/vcpu: Remove struct vcpu allocation restriction when possible

x86/vcpu: Remove struct vcpu allocation restriction when possible

There is no need for struct vcpu to live below the 4G boundary for PV guests,
or for HVM vcpus using HAP.

Plumb struct domain into alloc_vcpu_struct() so the x86 version can query the
domain's type and paging settings.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>