nvmm: Port to DragonFly #18: kernel memory allocation

nvmm: Port to DragonFly #18: kernel memory allocation

Use kmem_alloc() and kmem_free() to implement uvm_km_alloc() and
uvm_km_free() as they're used in svm_vcpu_create() and vmx_vcpu_create().
However, our kmem_alloc() may return 0 (i.e., allocation failure), so
need an extra check in the caller functions.

Since we've defined 'kmem_alloc' and 'kmem_free' macros to adapt
NetBSD's functions to use our kmalloc() and kfree().  Therefore, extra
parentheses are added around 'kmem_alloc' and 'kmem_free' to avoid macro
expansion, so the original functions would be called.

In addition, change the 'kmem_free()' to 'uvm_km_free()' in
vmx_vcpu_create(), aligning with the invocation pattern as well as
the use case in svm_vcpu_create().