libxl: suspend: New domain_suspend_pvcontrol_acked

libxl: suspend: New domain_suspend_pvcontrol_acked

Factor out domain_suspend_pvcontrol_acked.

This replaces a bunch of open-coded strcmp()s and makes the code
clearer.  It also eliminates the need to check for state==NULL each
time it's read, because we can check for NULL once before the strcmp.

No functional change.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v3: Improve comment re xswatch state ENOENT