kvm: Move kvm_allows_irq0_override() to target-i386, fix return type

kvm: Move kvm_allows_irq0_override() to target-i386, fix return type

kvm_allows_irq0_override() is a totally x86 specific concept:
move it to the target-specific source file where it belongs.
This means we need a new header file for the prototype:
kvm_i386.h, in line with the existing kvm_ppc.h.

While we are moving it, fix the return type to be 'bool' rather
than 'int'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Avi Kivity <avi@redhat.com>