isa: introduce wrapper isa_connect_gpio_out

Currently a direct access to the device structure field is used to connect ISA
device IRQ to the bus. GPIO access should be used instead if possible.

The patch adds wrapper isa_connect_gpio_out. The function connects specified
output GPIO to specified ISA IRQ.

Signed-off-by: Efimov Vasily <real@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>