KVM: MIPS: Use kvm_vcpu_cache to allocate vCPUs

For reasons unknown, MIPS configures the vCPU allocation cache but
allocates vCPUs via kzalloc().  Allocate from the vCPU cache in
preparation for moving vCPU allocation to common KVM code.

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