OvmfPkg/ResetSystemLib: factor out ResetShutdown()

Move the ResetShutdown() definition to its own file. This will help us
introduce:

- a new library instance that is not broken in runtime modules (the
  current library instance is broken in runtime modules),

- another new library instance for bhyve support.

While at it, squash AcpiPmControl() into ResetShutdown(), open-coding
SuspendType=0. This is justified because we've had no other callers for
AcpiPmControl() since commit 2d9950a2 ("OvmfPkg: remove
EnterS3WithImmediateWake () from ResetSystemLib", 2020-01-10).

Tested with the "reset -s" UEFI shell command, on both i440fx and q35.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2675


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200417153751.7110-5-lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Rebecca Cran <rebecca@bsdio.com>