This reverts commit 61f2b6ba and most of
commit d8916dc8, which effectively
brings back commit ef1065cf written by
Jim Fehlig:

The qemu migration speed default is 32MiB/s as defined in migration.c

/* Migration speed throttling */
static int64_t max_throttle = (32 << 20);

There's no need to throttle migration when targeting a file, so set
migration speed to unlimited prior to migration, and restore to libvirt
default value after migration.

Default units is MB for migrate_set_speed monitor command, so
(INT64_MAX / (1024 * 1024)) is used for unlimited migration speed.

This was reverted because migration to file could not be canceled and
even monitored since qemu was not processing any monitor commands until
the migration finished. This is now different as we make sure the
file descriptor we pass to qemu is able to properly report EAGAIN.
Recent qemu changes might have helped as well.

I tested managedsave with this patch in and indeed, it is 10x faster
while I can still monitor its progress.