KVMHypervisor: return memory and cpus as integers

Currently the KVM hypervisor returns strings for the memory and cpu
values, while the xen hypervisor returns integers. Making this uniform
converting the values to integers in KVM as well.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>