xhci: add support for suspend/resume

The OS can ask the xhci controller to save and restore its
internal state, which is used by the OS when the system is
suspended and resumed.

This patch handles writes to the save + restore bits in the
command register.  Only thing it does is updating the
restore error bit in the status register to signal an error
on restore.  The guest OS should do a full reinitialization
after resume then.

This is the minimal patch which gets S3 going with xhci.
Implementing full save/restore support is TBD.

https://bugzilla.redhat.com/show_bug.cgi?id=1012365

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
