Improve robustness of the Xen balloon driver.
sys/dev/xen/balloon/balloon.c:
Remove unused and commented out code.
Fix deadlock caused by performing a sleepable malloc
while holding the balloon mutex.
Perform proper accounting of the memory used by the domain.
Submitted by: Roger Pau Monné
Sponsored by: Citrix Systems R&D
Reviewed by: gibbs
Approved by: gibbs (mentor)
Approved by: re (gjb)
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