x86/reboot: emergency callbacks are now registered by common KVM code

Guard them with CONFIG_KVM_X86_COMMON rather than the two vendor modules.
In practice this has no functional change, because CONFIG_KVM_X86_COMMON
is set if and only if at least one vendor-specific module is being built.
However, it is cleaner to specify CONFIG_KVM_X86_COMMON for functions that
are used in kvm.ko.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Fixes: 590b09b1 ("KVM: x86: Register "emergency disable" callbacks when virt is enabled")
Fixes: 6d55a942 ("x86/reboot: Unconditionally define cpu_emergency_virt_cb typedef")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>