After the transition to Meson, the build directory now have
subdirectories named "qemu-system-*.p", and device-crash-test
will try to execute them as if they were binaries.  This results
in errors like:

  PermissionError: [Errno 13] Permission denied: './qemu-system-or1k.p'

When generating the default list of binaries to test, check if
the path is actually a file and if it's executable.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20201026125238.2752882-1-ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>