KVM: X86: Add the Document for KVM_CAP_X86_BUS_LOCK_EXIT

Introduce a new capability named KVM_CAP_X86_BUS_LOCK_EXIT, which is
used to handle bus locks detected in guest. It allows the userspace to
do custom throttling policies to mitigate the 'noisy neighbour' problem.

Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Message-Id: <20201106090315.18606-5-chenyi.qiang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>