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
MFC after:	2 days