KVM: x86: Treat x2APIC's ICR as a 64-bit register, not two 32-bit regs

Emulate the x2APIC ICR as a single 64-bit register, as opposed to forking
it across ICR and ICR2 as two 32-bit registers.  This mirrors hardware
behavior for Intel's upcoming IPI virtualization support, which does not
split the access.

Previous versions of Intel's SDM and AMD's APM don't explicitly state
exactly how ICR is reflected in the vAPIC page for x2APIC, KVM just
happened to speculate incorrectly.

Handling the upcoming behavior is necessary in order to maintain
backwards compatibility with KVM_{G,S}ET_LAPIC, e.g. failure to shuffle
the 64-bit ICR to ICR+ICR2 and vice versa would break live migration if
IPI virtualization support isn't symmetrical across the source and dest.

Cc: Zeng Guang <guang.zeng@intel.com>
Cc: Chao Gao <chao.gao@intel.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220204214205.3306634-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>