mdb-bhyve mishandles memseg offsets
The library shim over libvmmapi which mdb uses to debug bhyve instances (libvmm) is in charge of mapping in the various memory segments allocated to the instance in order for mdb to inspect and manipulate them. When this was originally implemented, memory segments were never mapped in from an offset (where the start of the mapping in the VM does not correspond to the start of the memseg). As of #12792, this changed for the bootrom segment, which has lead to several challeges for mdb-bhyve (see also: #13008). Since such mapping offsets can be (and are now) a reality,
vmm_map should properly handle them.