With some upcoming patches to save/restore the Hyper-V drivers related
states, a Linux VM running on Hyper-V will be able to hibernate. When
a Linux VM hibernates, unluckily we must disable the memory hot-add/remove
and balloon up/down capabilities in the hv_balloon driver
(drivers/hv/hv_balloon.c), because these can not really work according to
the design of the related back-end driver on the host.

By default, Hyper-V does not enable the virtual ACPI S4 state for a VM;
on recent Hyper-V hosts, the administrator is able to enable the virtual
ACPI S4 state for a VM, so we hope to use the presence of the virtual ACPI
S4 state as a hint for hv_balloon to disable the aforementioned
capabilities. In this way, hibernation will work more reliably, from the
user's perspective.

By marking acpi_sleep_state_supported() non-static, we'll be able to
implement a hv_is_hibernation_supported() API in the always-built-in
module arch/x86/hyperv/hv_init.c, and the API will be called by hv_balloon.
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>