hw: Fix qemu_allocate_irqs() leaks

Replace qemu_allocate_irqs(foo, bar, 1)[0]
with qemu_allocate_irq(foo, bar, 0).

This avoids leaking the dereferenced qemu_irq *.

Cc: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
[PC Changes:
 * Applied change to instance in sh4/sh7750.c
]
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Kirill Batuzov <batuzovk@ispras.ru>
[AF: Fix IRQ index in sh4/sh7750.c]
Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>