KVM: x86: believe what KVM says about WAITPKG

KVM: x86: believe what KVM says about WAITPKG

Currently, QEMU is overriding KVM_GET_SUPPORTED_CPUID's answer for
the WAITPKG bit depending on the "-overcommit cpu-pm" setting.  This is a
bad idea because it does not even check if the host supports it, but it
can be done in x86_cpu_realizefn just like we do for the MONITOR bit.

This patch moves it there, while making it conditional on host
support for the related UMWAIT MSR.

Cc: qemu-stable@nongnu.org
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>