Previously, PartitionTable was constructed using a Kernel::StorageDevice
making it only usable in the kernel. This commit adds a new constructor
that takes a Core::File instead, making it usable in userland as well.
This also adds the m_block_size field which stores the block size of the
underlying device obtained by calling StorageDevice::block_size() in the
kernel or by using the STORAGE_DEVICE_GET_BLOCK_SIZE ioctl in userland.
This avoids the need for an #ifdef every time block size is needed.
