osstest: install irqbalance

osstest: install irqbalance

Or else all interrupts will get bound to (v)CPU 0.

This doesn't cause issues on small boxes, but boxes with a non-trivial
amount of CPUs can struggle without interrupts being balanced across
available vCPUs, as the number of vCPUs offered to dom0 matches the
number of physical CPUs.

For example sabro boxes (Xeon Silver 4114 x 2 sockets) would sometimes
report timeouts which seem to be solved by using irqbalance in dom0.

irqbalance is also available on Arm, so install unconditionally.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>