kvm: i386: require KVM_CAP_ADJUST_CLOCK

This was introduced in KVM in Linux 2.6.33, we can require it
unconditionally.  KVM_CLOCK_TSC_STABLE was only added in Linux 4.9,
for now do not require it (though it would allow the removal of some
pretty yucky code).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>