dma: do not depend on kvm_enabled()

Memory barriers are needed also by Xen and, when the ioeventfd
bugs are fixed, by TCG as well.

sysemu/kvm.h is not anymore needed in sysemu/dma.h, move it to
the actual users.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/misc/pci-testdev.c

@@ -21,6 +21,7 @@
 #include "hw/hw.h"
 #include "hw/pci/pci.h"
 #include "qemu/event_notifier.h"
+#include "sysemu/kvm.h"
 
 typedef struct PCITestDevHdr {
     uint8_t test;