Turns out that it's not enough to pass the qemu:///embed root to
virQEMUDriverConfigNew(), you also have to make sure the same
string is copied into the virQEMUDriver structure yourself, and
not doing so in our case resulted in the cleanup never happening
and in distcheck failing because of that.

On the other hand, actually setting config->embeddedRoot would
result in different paths being generated for each test run, which
would obviously break qemuxml2argvtest, so that's not an option
either.

This reverts commit d98cc196.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>