x86/msr: Introduce and use default MSR policies

x86/msr: Introduce and use default MSR policies

For now, the default and max policies remain identical, but this will change
in the future.

Update XEN_SYSCTL_get_cpu_policy and init_domain_msr_policy() to use the
default policies.

Take the opportunity sort PV ahead of HVM, as is the prevailing style
elsewhere.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>