KVM: Add parameter "kvm" to kvm_cpu_dirty_log_size() and its callers

Add a parameter "kvm" to kvm_cpu_dirty_log_size() and down to its callers:
kvm_dirty_ring_get_rsvd_entries(), kvm_dirty_ring_alloc().

This is a preparation to make cpu_dirty_log_size a per-VM value rather than
a system-wide value.

No function changes expected.

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>