add -drive if=none

This adds a host drive, but doesn't implicitly add a guest drive for it.
First step in splitting host and guest configuration, check the
following patches to see how this can be used ...
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>
Message-Id: 

sysemu.h

@@ -144,6 +144,7 @@ extern unsigned int nb_prom_envs;
 #endif
 
 typedef enum {
+    IF_NONE,
     IF_IDE, IF_SCSI, IF_FLOPPY, IF_PFLASH, IF_MTD, IF_SD, IF_VIRTIO, IF_XEN,
     IF_COUNT
 } BlockInterfaceType;

vl.c

@@ -1982,6 +1982,9 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
 	} else if (!strcmp(buf, "xen")) {
 	    type = IF_XEN;
             max_devs = 0;
+	} else if (!strcmp(buf, "none")) {
+	    type = IF_NONE;
+            max_devs = 0;
 	} else {
             fprintf(stderr, "qemu: unsupported bus type '%s'\n", buf);
             return NULL;
@@ -2195,6 +2198,7 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
     case IF_PFLASH:
     case IF_MTD:
     case IF_VIRTIO:
+    case IF_NONE:
         break;
     case IF_COUNT:
         abort();