qemu-iotests: let "check" spawn an arbitrary test command

qemu-iotests: let "check" spawn an arbitrary test command

Right now there is no easy way for "check" to print a reproducer command.
Because such a reproducer command line would be huge, we can instead teach
check to start a command of our choice.  This can be for example a Python
unit test with arguments to only run a specific subtest.

Move the trailing empty line to print_env(), since it always looks better
and one caller was not adding it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Tested-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20210323181928.311862-5-pbonzini@redhat.com>
Message-Id: <20210503110110.476887-5-pbonzini@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>