Clear mem_path if we fall back to anonymous RAM allocation

Clear mem_path if we fall back to anonymous RAM allocation

If the -mem-path option is set, we attempt to map the guest's RAM from a
file in the given path; it's usually used to back guest RAM with hugepages.
If we're unable to (e.g. not enough free hugepages) then we fall back to
allocating normal anonymous pages.  This behaviour can be surprising, but a
comment in allocate_system_memory_nonnuma() suggests it's legacy behaviour
we can't change.

What really isn't ok, though, is that in this case we leave mem_path set.
That means functions which attempt to determine the pagesize of main RAM
can erroneously think it is hugepage based on the requested path, even
though it's not.

This is particular bad for the pseries machine type.  KVM HV limitations
mean the guest can't use pagesizes larger than the host page size used to
back RAM.  That means that such a fallback, rather than merely giving
poorer performance than expected will cause the guest to freeze up early in
boot as it attempts to use large page mappings that can't work.

This patch addresses the problem by clearing the mem_path variable when we
fall back to anonymous pages, meaning that subsequent attempts to
determine the RAM page size will get an accurate result.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>