pmxcfs: get guest cfg properties: use g_string_sized_new

pmxcfs: get guest cfg properties: use g_string_sized_new

While with NULL as first argument g_string_new_len effectively
becomes a g_string_sized_new it can be confusing as the docs do not
mention that. Also this may lead to an error if one changes the call
with out to much research, so fix it to the one function we should
used to begin with here.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>