KVM: VMX: Ignore userspace MSR filters for x2APIC

Rework the resetting of the MSR bitmap for x2APIC MSRs to ignore userspace
filtering.  Allowing userspace to intercept reads to x2APIC MSRs when
APICV is fully enabled for the guest simply can't work; the LAPIC and thus
virtual APIC is in-kernel and cannot be directly accessed by userspace.
To keep things simple we will in fact forbid intercepting x2APIC MSRs
altogether, independent of the default_allow setting.

Cc: Alexander Graf <graf@amazon.com>
Cc: Aaron Lewis <aaronlewis@google.com>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20201005195532.8674-3-sean.j.christopherson@intel.com>
[Modified to operate even if APICv is disabled, adjust documentation. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>