Before the guest finishes the device initialization, the device can be
removed anytime by the host, and after that the host won't respond to
the guest's request, so the guest should be prepared to handle this
case.

Add a polling mechanism to detect device presence.
Signed-off-by: NDexuan Cui <decui@microsoft.com>
[lorenzo.pieralisi@arm.com: edited commit log]
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NHaiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>