update_iface_link_state: make host_iface configurable

[smitterl]

Need to run this test against the second interface on our host.

[smitterl]

@chloerh @yanqzhan please help review

[smitterl]

# avocado run --test-runner runner --vt-type libvirt --vt-machine-type s390-virtio --vt-connect-uri qemu:///system virtual_network.update_device.iface_link_state.ethernet.virtio.update --vt-extra-params "host_iface = enc2"
No python imaging library installed. Screendump and Windows guest BSOD detection are disabled. In order to enable it, please install python-imaging or the equivalent for your distro.
No python imaging library installed. PPM image conversion to JPEG disabled. In order to enable it, please install python-imaging or the equivalent for your distro.
JOB ID     : 58bf45b0d30f4fc295979af9c00fbd33ee0c0c77
JOB LOG    : /var/log/avocado/job-results/job-2024-09-04T10.39-58bf45b/job.log
 (1/1) type_specific.io-github-autotest-libvirt.virtual_network.update_device.iface_link_state.ethernet.virtio.update: PASS (47.00 s)
RESULTS    : PASS 1 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0
JOB HTML   : /var/log/avocado/job-results/job-2024-09-04T10.39-58bf45b/results.html
JOB TIME   : 47.43 s