rust: prefer importing std::ptr over core::ptr

rust: prefer importing std::ptr over core::ptr

The std::ptr is same as core::ptr, but std has already been used in many
cases and there's no need to choose non-std library.

So, use std::ptr directly to make the used ptr library as consistent as
possible.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250218080835.3341082-1-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>