kvm: Introduce support for -mem-path

Using hugepages, KVM instances can get a good performance boost. To
activate that, we need to pass the -mem-path argument to KVM along with
the mount point of the hugetlbfs file system on the node.

For the sake of memory availability computation, we use the -mem-prealloc
argument when enabling hugepages, so KVM will reserve all hugepages it
needs when it starts. This avoids allocating an instance on a node that
will not have enough pages in case other instance needs more than what
is available after it boots.

Signed-off-by: Miguel Di Ciurcio Filho <miguel.filho@gmail.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>