hw/i386/pc: Ensure vmport prerequisites are fulfilled

hw/i386/pc: Ensure vmport prerequisites are fulfilled

Since commit 4ccd5fe22feb95137d325f422016a6473541fe9f ('pc: add option
to disable PS/2 mouse/keyboard'), the vmport will not be created unless
the i8042 PS/2 controller is enabled. To avoid confusion, let's fail if
vmport was explicitly requested, but the i8042 controller is disabled.
This also changes the behavior of vmport=auto to take i8042 controller
availability into account.

Signed-off-by: Kamil Szczęk <kamil@szczek.dev>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Message-ID: <0MS3y5E-hHqODIhiuFxmCnIrXd612JIGq31UuMsz4KGCKZ_wWuF-PHGKTRSGS0nWaPEddOdF4YOczHdgorulECPo792OhWov7O9BBF6UMX4=@szczek.dev>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>