qemu: Fix utils.system_output() calls incompatible with at 0.14

In some places of our code, we're calling utils.system_output
with a parameter verbose, introduced in autotest 0.15. Therefore,
for people using the Fedora/RHEL shipped RPMs, a bug will
happen since the API on their RPMs doesn't know about the verbose
param.

Therefore, remove such calls and get the equivalent construction
using the utils.run API, that does support the parameter.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>