Add a new API virDomainShutdownFlags and define:

    VIR_DOMAIN_SHUTDOWN_DEFAULT        = 0,
    VIR_DOMAIN_SHUTDOWN_ACPI_POWER_BTN = (1 << 0),
    VIR_DOMAIN_SHUTDOWN_GUEST_AGENT    = (1 << 1),

Also define some flags for the reboot API

    VIR_DOMAIN_REBOOT_DEFAULT        = 0,
    VIR_DOMAIN_REBOOT_ACPI_POWER_BTN = (1 << 0),
    VIR_DOMAIN_REBOOT_GUEST_AGENT    = (1 << 1),

Although these two APIs currently have the same flags, using
separate enums allows them to expand separately in the future.

Add stub impls of the new API for all existing drivers