KVM: VMX: re-add ple_gap module parameter

Apparently, the ple_gap parameter was accidentally removed
by commit c8e88717. Add it
back.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Cc: stable@vger.kernel.org
Fixes: c8e88717
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>