gdb: fix "osv mmap" and friends

It appears that in GDB, (mmu::vma*)0 does not work, and one needs to enclose
the type's name in single quotes: ('mmu::vma'*)0. This broke the vma_list
function in scripts/loader.py, and caused an exception in "osv mmap" and
other commands using the vma_list function.

This patch adds the missing single-quotes.

I don't understand how this code ever worked for anybody...
I'm using gdb-7.6.1 from Fedora 19, if it matters.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>