KVM: x86: build kvm_userspace_memory_region in x86_set_memory_region

KVM: x86: build kvm_userspace_memory_region in x86_set_memory_region

The next patch will make x86_set_memory_region fill the
userspace_addr.  Since the struct is not used untouched
anymore, it makes sense to build it in x86_set_memory_region
directly; it also simplifies the callers.

Reported-by: Alexandre DERUMIER <aderumier@odiso.com>
Cc: stable@vger.kernel.org
Fixes: fa6d11aebd472c25d64c3372bd54f274d9fece48
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>