KVM: VMX: Use accessors for GPRs outside of dedicated caching logic

... now that there is no overhead when using dedicated accessors.

Opportunistically remove a bogus "FIXME" in handle_rdmsr() regarding
the upper 32 bits of RAX and RDX.  Zeroing the upper 32 bits is
architecturally correct as 32-bit writes in 64-bit mode unconditionally
clear the upper 32 bits.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>