config: move x86 kvm_guest.config to a common location

kvm_guest.config is useful for KVM guests on other arches, and nothing
in it appears to be x86 specific, so just move the whole file. Kbuild
will find it in either location.
Signed-off-by: NRob Herring <robh@kernel.org>
Cc: Christoffer Dall <christoffer.dall@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: kvmarm@lists.cs.columbia.edu
Cc: kvm@vger.kernel.org
Acked-by: NChristoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>