kvmvapic: Disable if there is insufficient memory

kvmvapic: Disable if there is insufficient memory

We need at least 1M of RAM to map the option ROM. Otherwise, we will
corrupt host memory or even crash:

    $ qemu-system-x86_64 -nodefaults --enable-kvm -vnc :0 -m 640k
    Segmentation fault (core dumped)

Reported-and-tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
(cherry picked from commit a9605e0317c7a6d5e68f3a3b6708c8ef1096f4bc)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>