libxl: add support for specifying clock offset and adjustment

libxl: add support for specifying clock offset and adjustment

libxl supports setting the domain real time clock to local time or
UTC via the localtime field of libxl_domain_build_info. Adjustment
of the clock is also supported via the rtc_timeoffset field. The
libvirt libxl driver has never supported these settings, instead
relying on libxl's default of a UTC real time clock with adjustment
set to 0.

There is at least one user that would like the ability to change
the defaults

https://www.redhat.com/archives/libvirt-users/2018-February/msg00059.html

Add support for specifying a local time clock and for specifying an
adjustment for both local time and UTC clocks. Add a test case to
verify the XML to libxl_domain_config conversion.

Local time clock and clock adjustment is already supported by the
XML <-> xl.cfg converter. What is missing is an explicit test for
the conversion. There are plenty of existing tests that all use UTC
with 0 adjustment. Hijack test-fullvirt-tsc-timer to test a local
time clock with 1 hour adjustment.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>