Rearrange block headers

Changing block.h or blockdev.h resulted in recompiling most objects.

Move DriveInfo typedef and BlockInterfaceType enum definitions
to qemu-common.h and rearrange blockdev.h use to decrease churn.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/xen_machine_pv.c b/hw/xen_machine_pv.c
index 586214d..77a34bf 100644
--- a/hw/xen_machine_pv.c
+++ b/hw/xen_machine_pv.c
@@ -28,6 +28,7 @@
 #include "boards.h"
 #include "xen_backend.h"
 #include "xen_domainbuild.h"
+#include "blockdev.h"
 
 static void xen_init_pv(ram_addr_t ram_size,
 			const char *boot_device,