lib/vmalloc: fix potential race and non-standard pointer arithmetic

The pointer vfree_top should only be accessed with the lock held, so
make sure we return a local copy of the pointer taken safely inside the
lock.

Also avoid doing pointer arithmetic on void pointers. Gcc allows it but
it is ugly. Use uintptr_t for doing maths on the pointer.

This will also come useful in upcoming patches.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200622162141.279716-7-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>