test: let qemuhotplugtest report details of init fails

test: let qemuhotplugtest report details of init fails

If virHostdevManagerGetDefault in qemuhotplugtest fails it works
for quite a while to later segfault when accessing
mgr->activePCIHostdevs.

Report the error details and break on a failed init to see the
real issue right away.

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>