storage: fallback kernels_dir path if there is no 'kernel' volume
storage: fallback kernels_dir path if there is no 'kernel' volume

Return meaningful value for kernels_dir if VM has no 'kernel' volume.
Right now it's mostly useful for tests, but could be also used for new
VM classes which doesn't have modules.img, but still use dom0-provided
kernel.

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin