migration: Fix file migration with fdset

migration: Fix file migration with fdset

When the "file:" migration support was added we missed the special
case in the qemu_open_old implementation that allows for a particular
file name format to be used to refer to a set of file descriptors that
have been previously provided to QEMU via the add-fd QMP command.

When using this fdset feature, we should not truncate the migration
file because being given an fd means that the management layer is in
control of the file and will likely already have some data written to
it. This is further indicated by the presence of the 'offset'
argument, which indicates the start of the region where QEMU is
allowed to write.

Fix the issue by replacing the O_TRUNC flag on open by an ftruncate
call, which will take the offset into consideration.

Fixes: 385f510df5 ("migration: file URI offset")
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>