x86/msr: Rename the msr policy objects

x86/msr: Rename the msr policy objects

After attempting to develop the infrastructure, it turns out that the choice
of naming is suboptimal.

Rename msr_domain_policy to just msr_policy to mirror the CPUID side of
things, and alter the 'dp' variable name convention to 'mp'.  While altering
all the names, export all of the system msr_policy objects (which are already
global symbols).

Rename msr_vcpu_policy to vcpu_msrs and switch 'vp' to 'msrs' in code.  Update
the arch_vcpu field name to match.

No functional change.

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