rust: use std::os::raw instead of core::ffi

rust: use std::os::raw instead of core::ffi

core::ffi::c_* types were introduced in Rust 1.64.0.  Use the older types
in std::os::raw, which are now aliases of the types in core::ffi.  There is
no need to compile QEMU as no_std, so this is acceptable as long as we support
a version of Debian with Rust 1.63.0.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>