tests/qemu-iotests: Rework the checks and spots using GNU sed

Instead of failing the iotests if GNU sed is not available (or skipping
them completely in the check-block.sh script), it would be better to
simply skip the bash-based tests that rely on GNU sed, so that the other
tests could still be run. Thus we now explicitely use "gsed" (either as
direct program or as a wrapper around "sed" if it's the GNU version)
in the spots that rely on the GNU sed behavior. Statements that use the
"-r" parameter of sed have been switched to use "-E" instead, since this
switch is supported by all sed versions on our supported build hosts
(most also support "-r", but macOS' sed only supports "-E"). With all
these changes in place, we then can also remove the sed checks from the
check-block.sh script, so that "make check-block" can now be run on
systems without GNU sed, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220216125454.465041-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>