iotests: Consistent $IMGOPTS boundary matching

iotests: Consistent $IMGOPTS boundary matching

To disallow certain refcount_bits values, some _unsupported_imgopts
invocations look like "refcount_bits=1[^0-9]", i.e. they match an
integer boundary with [^0-9].  This expression does not match the end of
the string, though, so it breaks down when refcount_bits is the last
option (which it tends to be after the rewrite of the check script in
Python).

Those invocations could use \b or \> instead, but those are not
portable.  They could use something like \([^0-9]\|$\), but that would
be cumbersome.  To make it simple and keep the existing invocations
working, just let _unsupported_imgopts match the regex against $IMGOPTS
plus a trailing space.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210210095128.22732-1-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>