KVM: Provide helper to get kvm dirty log

Provide a helper kvm_slot_get_dirty_log() to make the function
kvm_physical_sync_dirty_bitmap() clearer.  We can even cache the as_id
into KVMSlot when it is created, so that we don't even need to pass it
down every time.

Since at it, remove return value of kvm_physical_sync_dirty_bitmap()
because it should never fail.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210506160549.130416-5-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>