KVM: selftests: Make vm_create() a wrapper that specifies VM_MODE_DEFAULT

Add ____vm_create() to be the innermost helper, and turn vm_create() into
a wrapper the specifies VM_MODE_DEFAULT.  Most of the vm_create() callers
just want the default mode, or more accurately, don't care about the mode.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>