Fix warnings/errors when building vmm.ko with gcc:

• fix warning about comparison of 'uint8_t v_tpr >= 0' always being true.

• fix error triggered by an empty clobber list in the inline assembly for "clgi" and "stgi"

• fix error when compiling "vmload %rax", "vmrun %rax" and "vmsave %rax". The gcc assembler does not like the explicit operand "%rax" while the clang assembler requires specifying the operand "%rax". Fix this by encoding the instructions using the ".byte" directive.

Reported by: julian
MFC after: 1 week