x86/pv: Simplify emulation for the 64bit base MSRs

x86/pv: Simplify emulation for the 64bit base MSRs

is_pv_32bit_domain() is an expensive predicate, but isn't used for speculative
safety in this case.  Swap to checking the Long Mode bit in the CPUID policy,
which is the architecturally correct behaviour.

is_canonical_address() is currently moderately simple, but it will become more
complicated when 5-level support is added.  Rearrange write_msr() to collapse
the common checks.

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