kvm: x86: Introduce KVM_CAP_EXCEPTION_PAYLOAD

This is a per-VM capability which can be enabled by userspace so that
the faulting linear address will be included with the information
about a pending #PF in L2, and the "new DR6 bits" will be included
with the information about a pending #DB in L2. With this capability
enabled, the L1 hypervisor can now intercept #PF before CR2 is
modified. Under VMX, the L1 hypervisor can now intercept #DB before
DR6 and DR7 are modified.

When userspace has enabled KVM_CAP_EXCEPTION_PAYLOAD, it should
generally provide an appropriate payload when injecting a #PF or #DB
exception via KVM_SET_VCPU_EVENTS. However, to support restoring old
checkpoints, this payload is not required.

Note that bit 16 of the "new DR6 bits" is set to indicate that a debug
exception (#DB) or a breakpoint exception (#BP) occurred inside an RTM
region while advanced debugging of RTM transactional regions was
enabled. This is the reverse of DR6.RTM, which is cleared in this
scenario.

This capability also enables exception.pending in struct
kvm_vcpu_events, which allows userspace to distinguish between pending
and injected exceptions.

Reported-by: Jim Mattson <jmattson@google.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>