oeqa/utils/dump: Add default commands and directory

oeqa/utils/dump: Add default commands and directory

Currently if qemu fails when running a selftest and
tries to run some commands on the host it will fail
because some variables required by the Dumper class
do not exist because testimage was not included.

This change adds a default parent directory to save
the dumps for the host or target. Also adds default
commands to run if no commands were provided to the
class. With these changes the previous errors using
selftest don't show anymore.

[YOCTO #8306]

Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>