vhost-user: Fix VHOST_SET_MEM_TABLE processing

vhost-user: Fix VHOST_SET_MEM_TABLE processing

qemu_get_ram_fd doesn't accept a guest physical address. ram_addr_t are
opaque values that are assigned in qemu_ram_alloc.

Find the ram_addr_t corresponding to the userspace_addr using qemu_ram_addr_from_host,
and then call qemu_get_ram_fd on it.

Thanks to Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>