block/vpc: Add a sanity check that fixed-size images have the right type

The code in vpc.c uses BDRVVPCState->footer.type in various places
to decide whether the image is a fixed-size (VHD_FIXED) or a dynamic
(VHD_DYNAMIC) image. However, we never check that this field really
contains VHD_FIXED if we detected a fixed size image in vpc_open(),
so a wrong value here could cause quite some trouble during runtime.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211012082702.792259-1-thuth@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>