subprojects: add the anyhow crate

This is a standard replacement for Box<dyn Error> which is more efficient (it only
occcupies one word) and provides a backtrace of the error. This could be plumbed

subprojects: add the anyhow crate

This is a standard replacement for Box<dyn Error> which is more efficient (it only
occcupies one word) and provides a backtrace of the error. This could be plumbed
into &error_abort in the future.

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

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