Page table creation clobbers provided command line

[josephlr]

Both cloud-hypervisor and firecracker support passing a command line argument to the image they are launching. The firmware code in bzimage.rs tries to create a command line that consists of:

• The hypervisor-provided command line, followed by
• The command line from the on-disk data

However, the provided command line is often at address 0x20000 (cloud-hypervisor example and firecracker example). This causes a problem when running setup_pagetables() which writes a single PML3 table at 0xa000 but writes 64 PML2 tables starting at 0xb000, this will overwrite the command line with garbage, causing either:

• Nothing to be prepended to the command line when it should
• Junk to be prepended to the commandline

[rbradford]

@josephlr Two solutions come to mind:

1. Find somewhere safe to store the pagetables and just use that
2. Parse the command line very early (delaying expanding the identity mapping) and save anything we need and then clobber anyway.

[josephlr]

Fixed by #23 in de090ed