vmd(8): gracefully handle hitting data limits when starting a vm

vmd(8): gracefully handle hitting data limits when starting a vm

With recent changes to login.conf(5) to restrict daemon datasize
to a finite value, users can now hit resource limits when attempting
to start a vm.

This change fixes the error path when hitting the limit. vmd(8)
will no longer abort and memory error messages are relayed to the
user.

While here, address potential under-reads/writes using atomicio
when relaying data between the child vm process and vmd's vmm
process.

Original diff from tedu@. OK mlarkin@.