Make the --hypervisor argument optional in process_common_args().
When it is not supplied, detect_hypervisor() now selects the backend
from the host device node: /dev/mshv for MSHV, /dev/kvm for KVM, and
errors out when neither is present. An explicit --hypervisor still
overrides the detection.
Assisted-by: Claude:Opus-4.8
Signed-off-by: Muminul Islam <muislam@microsoft.com>