mshv: refactor synic init and cleanup

mshv: refactor synic init and cleanup

Rename mshv_synic_init() to mshv_synic_cpu_init() and
mshv_synic_cleanup() to mshv_synic_cpu_exit() to better reflect that
these functions handle per-cpu synic setup and teardown.

Use mshv_synic_init/cleanup() to perform init/cleanup that is not per-cpu.
Move all the synic related setup from mshv_parent_partition_init.

Move the reboot notifier to mshv_synic.c because it currently only
operates on the synic cpuhp state.

Move out synic_pages from the global mshv_root since its use is now
completely local to mshv_synic.c.

This is in preparation for adding more stuff to mshv_synic_init().

No functional change.

Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>