KVM: x86: Add WARNs to detect out-of-bounds register indices

Add WARN_ON_ONCE() checks in kvm_register_{read,write}() to detect reg
values that would cause KVM to overflow vcpu->arch.regs.  Change the reg
param to an 'int' to make it clear that the reg index is unverified.

Regarding the overhead of WARN_ON_ONCE(), now that all fixed GPR reads
and writes use dedicated accessors, e.g. kvm_rax_read(), the overhead
is limited to flows where the reg index is generated at runtime.  And
there is at least one historical bug where KVM has generated an out-of-
bounds access to arch.regs (see commit b68f3cc7

, "KVM: x86: Always
use 32-bit SMRAM save state for 32-bit kernels").

Adding the WARN_ON_ONCE() protection paves the way for additional
cleanup related to kvm_reg and kvm_reg_ex.

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