bhyve/snapshot: rename checkpoint_opcodes to be more generic

Generalize the naming here since the domain socket that uses these codes
might be used for purposes other than the save/restore feature.

• rename checkpoint_opcodes to ipc_opcode

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D28877

(cherry picked from commit b7fd9c4e5ebc69934205980d9f628ef8f21fb288)