iotests: Drop vpc from 004's and 104's format list

iotests: Drop vpc from 004's and 104's format list

Both tests require the test image to have a specific size; this cannot
be guaranteed by vpc (unless tuning the test specifically for that
format).

It is safe to exclude vpc from 004 because what is tested there is
implemented in a generic part in the block layer and not
format-specific.

It is safe to exclude vpc from 104 because for vpc basically every image
size is "unaligned", so if that would break at some point in time, we
would quickly notice just by running the generic tests.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>