kvm tools: Make kvm__arch_setup_firmware to return error code

kvm tools: Make kvm__arch_setup_firmware to return error code

If some of subsequent calls fails we better to return error
code instead of dying with a message. This is a first step
in getting rid of number of die() calls we have in code.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>