x86/VPMU: add public xenpmu.h

x86/VPMU: add public xenpmu.h

Add pmu.h header files, move various macros and structures that will be
shared between hypervisor and PV guests to it.

Move MSR banks out of architectural PMU structures to allow for larger sizes
in the future. The banks are allocated immediately after the context and
PMU structures store offsets to them.

While making these updates, also:
* Remove unused vpmu_domain() macro from vpmu.h
* Convert msraddr_to_bitpos() into an inline and make it a little faster by
  realizing that all Intel's PMU-related MSRs are in the lower MSR range.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Jan Beulich <jbeulich@suse.com>