docs/migration: add missing definitions to libxc-migration-stream

docs/migration: add missing definitions to libxc-migration-stream

The STATIC_DATA_END, X86_CPUID_POLICY and X86_MSR_POLICY record types have
sections explaining what they are but their values are not defined. Indeed
their values are defined as "Reserved for future mandatory records."

Also, the spec revision is adjusted to match the migration stream version
and an END record is added to the description of a 'typical save record for
and x86 HVM guest.'

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Fixes: 6f71b5b1506 ("docs/migration Specify migration v3 and STATIC_DATA_END")
Fixes: ddd273d8863 ("docs/migration: Specify X86_{CPUID,MSR}_POLICY records")
Acked-by: Wei Liu <wl@xen.org>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>