Rename default VM from vm1 to virt-tests-vm1

Using 'vm1' for the libvirt tests is a bit too generic IMO. Could
collide with a pre-existing VM, or just cause momentary user
confusion, as it did for me :)

Additionally remove all the instances where the old name was hardcoded
as a fallback. The QEMU tests don't use similar logic, and AFAICT it's
fair to assume that main_vm will always be specified, since it is
present in the shared base.cfg.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
libvirt: Disable many checks for domname tests

The domname tests largely only test virsh + libvirt API parameter
validation, there is very little qemu interraction. So it is largely
overkill to

- Encode a video file
- Verify that each guest is still alive after each test
- Unconditionally kill the VM after each test
- Run qemu-img check after each test

With these enabled, the default set of libvirt tests via ./run take
only about a minute on my machine, previously it was over 5 minutes.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
base.cfg: Document skip_image_processing

Signed-off-by: Cole Robinson <crobinso@redhat.com>