KVM: VMX: Use "illegal GPA" helper for PT/RTIT output base check

Use kvm_vcpu_is_illegal_gpa() to check for a legal GPA when validating a
PT output base instead of open coding a clever, but difficult to read,
variant.  Code readability is far more important than shaving a few uops
in a slow path.

No functional change intended.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200924194250.19137-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>