iotests: Replace IMGOPTS by _unsupported_imgopts

iotests: Replace IMGOPTS by _unsupported_imgopts

Some tests require compat=1.1 and thus set IMGOPTS='compat=1.1'
globally.  That is not how it should be done; instead, they should
simply set _unsupported_imgopts to compat=0.10 (compat=1.1 is the
default anyway).

This makes the tests heed user-specified $IMGOPTS.  Some do not work
with all image options, though, so we need to disable them accordingly.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Message-id: 20191107163708[email protected]
Signed-off-by: Max Reitz <[email protected]>