iotests: Do not suppress segfaults in bash tests

iotests: Do not suppress segfaults in bash tests

Currently, if a qemu/qemu-io/qemu-img/qemu-nbd invocation receives a
segmentation fault, that message is invisible in most cases since the
output is generally filtered and bash suppresses the segmentation fault
notice for any but the last element of a pipe.

Most of the time, the test will then fail anyway because of missing
output, but not necessarily (as happened with test 82 recently).

Fix this by making the corresponding environment variables point to
wrapper functions which execute the respective command in a subshell.

Giving options to qemu/qemu-io/qemu-img and path names with spaces were
broken for the Python tests; this patch "accidentally" fixes that.

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