kvm: Mark VCPU state dirty on creation

This avoids that early cpu_synchronize_state calls try to retrieve an
uninitialized state from the kernel. That even causes a deadlock if
io-thread is enabled.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
