vfs: unify virtual filesystems in registry and add /dev/vda to devfs

vfs: unify virtual filesystems in registry and add /dev/vda to devfs

- Added tmpfs_mount/tmpfs_kill_sb to tmpfs.c with proper cleanup
- Added devfs_mount/devfs_kill_sb to devfs.c (static globals, no cleanup needed)
- Added procfs_mount/procfs_kill_sb to procfs.c (static globals, no cleanup needed)
- Registered tmpfs, devfs, and procfs in filesystem type registry in init.c
- Added /dev/vda block device node to devfs for consistency with virtio-blk
- Updated headers (tmpfs.h, devfs.h, procfs.h) with VFS mount interface declarations
- Added necessary includes (fs.h, blockdev.h) to virtual filesystem implementations

Test results:
- Smoke test: 119/119 PASS
- Zero regressions