x86/hvm/rtc: preserved guest RTC offset during suspend/resume/migrate

x86/hvm/rtc: preserved guest RTC offset during suspend/resume/migrate

The emulated RTC is synchronized with the PV wallclock; any write to the
RTC will update struct domain's 'time_offset_seconds' field and call
update_domain_wallclock().

However, the value of 'time_offset_seconds' is not preserved in any save
record and indeed, when the RTC save record is loaded, the CMOS values
will be updated based on an offset value which may or may not have been
set by the toolstack [1]. This may result in making bogus values available
to the guest and messing up any calculations done in the call to
alarm_timer_update() at the end of rtc_load().

This patch extends the RTC save record to contain an offset value, which
will be zero filled on load of an older record. The 'time_offset_secoonds'
field in struct domain is also modified into a 'time_offset' struct,
containing a 'seconds' field and a boolean 'set' field.

The code in rtc_load() then uses the new value in the save record to
update the value of struct domain's 'time_offset.seconds' unless
'time_offset.set' is true, which will only be the case if the toolstack has
already performed a XEN_DOMCTL_settimeoffset.

[1] There is currently no way for a toolstack to read the value of
    'time_offset_seconds' from struct domain. In the past, any hope of
    preservation of the value across a guest life-cycle operation was based
    on relying on qemu-dm to write a value into xenstore whenever the RTC
    was updated, in response to an IOREQ with type IOREQ_TYPE_TIMEOFFSET
    being sent by Xen; see:

    https://xenbits.xen.org/gitweb/?p=qemu-xen-traditional.git;a=blob;f=i386-dm/helper2.c#l457

    but this behaviour was never forward-ported into upstream QEMU, which
    completely ignores that IOREQ type.
    In either case, nothing in xl or libxl ever samples the value of
    RTC offset from xenstore so any offset adjustment to a non-zero value
    performed by the guest (which in the case of Windows is highly likely
    as it normally writes RTC in local time, whereas Xen maintains time in
    UTC) is completely lost with the de-facto toolstack, and always has
    been. Instead, PV drivers are relied upon to paper over this gaping
    hole.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien@xen.org>