vl.c: prohibit simultaneous use of -icount with kvm or xen

With -icount, The vm_clock is updated with help from TCG (it counts
instructions at 2^ICOUNT ns/instructions). With KVM, the instruction
count is not available so KVM cannot provide this help.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>