KVM: x86/pmu: mask the result of rdpmc according to the width of the counters

[ Upstream commit 0e6f467ee28ec97f68c7b74e35ec1601bb1368a7 ]

This patch will simplify the changes in the next, by enforcing the
masking of the counters to RDPMC and RDMSR.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>