x86/HVM: fix initialization of wallclock time for PVHVM on migration

x86/HVM: fix initialization of wallclock time for PVHVM on migration

Call update_domain_wallclock_time on hvm_latch_shinfo_size even if
the bitness of the guest has already been set, this fixes the problem
with the wallclock not being set for PVHVM guests on resume from
migration.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Clean up the resulting code and retain the (slightly adjusted) original
comment.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>