The dynamic header's size is 1024 bytes.

vpc_open() reads only the 512 bytes of the dynamic header into buf[].
Works, because it doesn't actually access the second half.  However, a
colleague told me that GCC 11 warns:

    ../block/vpc.c:358:51: error: array subscript 'struct VHDDynDiskHeader[0]' is partly outside array bounds of 'uint8_t[512]' [-Werror=array-bounds]

Clean up to read the full header.

Rename buf[] to dyndisk_header_buf[] while there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201217162003.1102738-2-armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>