Some tests do not work well with certain refcount widths (i.e. you
cannot create internal snapshots with refcount_bits=1), so make those
widths unsupported.

Furthermore, add another filter to _filter_img_create in common.filter
which filters out the refcount_bits value.

This is necessary for test 079, which does actually work with any
refcount width, but invoking qemu-img directly leads to the
refcount_bits value being visible in the output; use _make_test_img
instead which will filter it out.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>