cpu_arm: Don't implement virCPUGetVendorForModel

cpu_arm: Don't implement virCPUGetVendorForModel

This patch is effectively a no-op, but I wanted to initialize
.getVendorForModel explicitly as implementing this function does not
even make sense on ARM. The CPU models in our CPU map are only used for
describing host CPU in capabilities XML and cannot be used for guest CPU
definition in domain XML anyway. The CPU models listed as supported in
domain capabilities XML are just passed through from QEMU.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>