fix saving and loading dirty bitmaps in snapshots

fix saving and loading dirty bitmaps in snapshots

Saving dirty bitmaps from our savevm-async code didn't work, since we
use a coroutine which holds the iothread mutex already (upstream savevm
is sync, migration uses a thread). Release the mutex before calling the
one function that (according to it's documentation) requires the lock to
*not* be held: qemu_savevm_state_pending.

Additionally, loading dirty bitmaps requires a call to
dirty_bitmap_mig_before_vm_start after "loadvm", which the upstream
savevm does explicitly afterwards - do that too.

This is exposed via the query-proxmox-support property
"pbs-dirty-bitmap-savevm".

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>