KVM: VMX: Rename vcpu_vmx's "nmsrs" to "nr_uret_msrs"

Rename vcpu_vmx.nsmrs to vcpu_vmx.nr_uret_msrs to explicitly associate
it with the guest_uret_msrs array.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923180409.32255-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>