iotests: Select a default machine for the rx and avr targets

iotests: Select a default machine for the rx and avr targets

If you are building only with either the new rx-softmmu or avr-softmmu
target, "make check-block" fails a couple of tests since there is no
default machine defined in these new targets. We have to select a machine
in the "check" script for these, just like we already do for the arm- and
tricore-softmmu targets.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200722161908.25383-1-thuth@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>