rust/util: use anyhow's native chaining capabilities

rust/util: use anyhow's native chaining capabilities

This simplifies conversions, making it possible to convert any error
into a QEMU util::Error with ".into()" (and therefore with "?").

The cost is having a separate constructor for when the error is a simple
string, but that is made easier by the ensure! macro.  If necessary,
another macro similar to "anyhow!" can be returned, but for now there
is no need for that.

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