tests: introduce GuestKernel wrapper

Introduce GuestKernel as the logical guest kernel artifact. It parses
the canonical vmlinux-<version>[-no-acpi] artifact name into version
and ACPI metadata. Discover the matching bzImage independently of
EFI eligibility, preserving no-ACPI variants and pairing debug
kernels with debug images. On aarch64 the Image boots both ways.

The guest_kernel fixture and pytest properties derive kernel identity
from the logical artifact instead of re-parsing file names; the fixture
still yields the vmlinux path and the reported identity is unchanged.

This prepares the vm_backend dimension, whose backends resolve a
GuestKernel to the concrete image they can boot.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>