kvmvapic: Catch invalid ROM size

If not caught early, a zero-length ROM will cause a NULL-pointer access
later on in patch_hypercalls when allocating a zero-length ROM copy and
trying to read from it.

CC: qemu-stable@nongnu.org
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>