qemu: command: Report stderr from qemu-bridge-helper

qemu: command: Report stderr from qemu-bridge-helper

There's a couple reports of things failing in this area (bug 1259070),
but it's tough to tell what's going wrong without stderr from
qemu-bridge-helper. So let's report stderr in the error message

Couple new examples:

virbr0 is inactive:
internal error: /usr/libexec/qemu-bridge-helper --use-vnet --br=virbr0 --fd=21: failed to communicate with bridge helper: Transport endpoint is not connected
stderr=failed to get mtu of bridge `virbr0': No such device

bridge isn't on the ACL:
internal error: /usr/libexec/qemu-bridge-helper --use-vnet --br=br0 --fd=21: failed to communicate with bridge helper: Transport endpoint is not connected
stderr=access denied by acl file