x86: Improve the efficiency of domain_relinquish_resources()

x86: Improve the efficiency of domain_relinquish_resources()

pci_release_devices() takes the global PCI lock.  Once pci_release_devices()
has completed, it will be called redundantly each time paging_teardown() and
vcpu_destroy_pagetables() continue.

This is liable to be millions of times for a reasonably sized guest, and is a
serialising bottleneck now that domain_kill() can be run concurrently on
different domains.

Instead of propagating the opencoding of the relinquish state machine, take
the opportunity to clean it up.

Leave a proper set of comments explaining that domain_relinquish_resources()
implements a co-routine.  Introduce a documented PROGRESS() macro to avoid
latent bugs such as the RELMEM_xen case, and make the new PROG_* states
private to domain_relinquish_resources().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>