rust: qemu-api-macros: add automatic TryFrom/TryInto derivation

rust: qemu-api-macros: add automatic TryFrom/TryInto derivation

This is going to be fairly common. Using a custom procedural macro
provides better error messages and automatically finds the right
type.

Note that this is different from the same-named macro in the
derive_more crate.  That one provides conversion from e.g. tuples
to enums with tuple variants, not from integers to enums.

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