drivers/virtualization: Add doorbell support for ivshmem driver

This basically adds support for an interrupt based ivshmem variant
called "ivshmem-doorbell".

This allows, via MSI-X, to get multiple vectors for notifications, get
assigned and ID and being able to send a message to another ID (another
VM).

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>