block/vpc: use current_size field for XenConverter VHD images

XenConverter VHD images are another VHD image where current_size is
different from the CHS values in the the format header.  Use
current_size as the default, by looking at the creator_app signature
field.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vpc.c

@@ -300,6 +300,7 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
      *      'win '  :  current_size     Hyper-V
      *      'd2v '  :  current_size     Disk2vhd
      *      'tap\0' :  current_size     XenServer
+     *      'CTXS'  :  current_size     XenConverter
      *
      *  The user can override the table values via drive options, however
      *  even with an override we will still use current_size for images
@@ -308,6 +309,7 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
     use_chs = (!!strncmp(footer->creator_app, "win ", 4) &&
                !!strncmp(footer->creator_app, "qem2", 4) &&
                !!strncmp(footer->creator_app, "d2v ", 4) &&
+               !!strncmp(footer->creator_app, "CTXS", 4) &&
                !!memcmp(footer->creator_app, "tap", 4)) || s->force_use_chs;
 
     if (!use_chs || bs->total_sectors == VHD_MAX_GEOMETRY || s->force_use_sz) {