Lift max ram slots limit in libvhost-user

Lift max ram slots limit in libvhost-user

Historically, VMs with vhost-user devices could hot-add memory a maximum
of 8 times. Now that the VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS
protocol feature has been added, VMs with vhost-user backends which
support this new feature can support a configurable number of ram slots
up to the maximum supported by the target platform.

This change adds VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS support for
backends built with libvhost-user, and increases the number of supported
ram slots from 8 to 32.

Memory hot-add, hot-remove and postcopy migration were tested with
the vhost-user-bridge sample.

Signed-off-by: Raphael Norwitz <[email protected]>
Message-Id: <1588533678[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>