KVM: x86/pmu: Treat "fixed" PMU type in RDPMC as index as a value, not flag

KVM: x86/pmu: Treat "fixed" PMU type in RDPMC as index as a value, not flag

Refactor KVM's handling of ECX for RDPMC to treat the FIXED modifier as an
explicit value, not a flag (minus one wart).  While non-architectural PMUs
do use bit 31 as a flag (for "fast" reads), architectural PMUs use the
upper half of ECX to encode the type.  From the SDM:

  ECX[31:16] specifies type of PMC while ECX[15:0] specifies the index of
  the PMC to be read within that type

Note, that the known supported types are 4000H and 2000H, i.e. look a lot
like flags, doesn't contradict the above statement that ECX[31:16] holds
the type, at least not by any sane reading of the SDM.

Keep the explicitly clearing of the FIXED "flag", as KVM subtly relies on
that behavior to disallow unsupported types while allowing the correct
indices for fixed counters.  This wart will be cleaned up in short order.

Opportunistically grab the per-type bitmask in the if-else blocks to
eliminate the one-off usage of the local "fixed" bool.

Reported-by: Jim Mattson <jmattson@google.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>