tools/kvm_stat: Fix kvm_exit filter name

The filter name is fixed to "exit_reason" for some kvm_exit events, no
matter what architect we have. Actually, the filter name ("exit_reason")
is only applicable to x86, meaning it's broken on other architects
including aarch64.

This fixes the issue by providing various kvm_exit filter names, depending
on architect we're on. Afterwards, the variable filter name is picked and
applied through ioctl(fd, SET_FILTER).

Reported-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Gavin Shan <gshan@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>