Than again, maybe not quite a bug. The quotes you supply are used-up by
the dom0 shell. This is expected.
Running the command with --pass-io, you can see that everything to the
right of & is run as a separate command on the target VM, except when
its escaped as \&.
qvm-run --pass-io untrusted "notify-send HI&THERE"
...results in "WHAT not found" in red lettering (from untrusted VM). But
using \& works as a single command.
You can also supply an additional set of quotes like this:
qvm-run --pass-io untrusted "notify-send \"HI&THERE\""
This quoting method seems mose usable because you don't have to be
vigilant about escaping different characters... just escaping the extra
quotes should do it.