rust: pl011: convert pl011_create to safe Rust

rust: pl011: convert pl011_create to safe Rust

Not a major change but, as a small but significant step in creating
qdev bindings, show how pl011_create can be written without "unsafe"
calls (apart from converting pointers to references).

This also provides a starting point for creating Error** bindings.

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