emul: Add an emulator for the eSPI Host

This emulator pretends a generic eSPI Host. It supports basic virtual
wires and port80 operations.

There are functions to trigger actions on the host side e.g. for
setting a virtual wire from the host to the eSPI slave, use
emul_espi_host_send_vw. It will prepare data and set a proper event
on the slave side which will trigger callback (if there is any).

Signed-off-by: Dawid Niedzwiecki <dn@semihalf.com>