OvmfPkg/IndustryStandard/Xen: Apply EDK2 coding style to XEN_VCPU_TIME_INFO

We are going to use new fields from the Xen headers. Apply the EDK2
coding style so that the code that is going to use it doesn't look out
of place.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2490


Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210412133003.146438-4-anthony.perard@citrix.com>