MFC r257769 to stable/10

Fix powerd/states on AMD cpus.  Resolves issues with system reporting:
hwpstate0: set freq failed, err 6

Tested on FX-8150 and others.

PR:		kern/167018
Submitted by:	avg@
Approved by:	re (gjb)
MFC 257876:

On XenServer the "halt" message is used instead of "poweroff", which
makes FreeBSD halt but not poweroff (as expected when issuing a
shutdown from the VM manager). Fix this by using the same handler
for both "halt" and "poweroff".

NB: The "halt" signal seems to be used on XenServer only.  The OSS
    Xen toolstack (xl) uses "poweroff" instead.

Submitted by:	Roger Pau Monné
Sponsored by:	Citrix Systems R&D
Reviewed by:	gibbs
Approved by:	gibbs (mentor)
Approved by:	re (gjb)
MFC 258176:

Fix accounting for hw.realmem on the i386 and amd64 platforms.

sys/i386/i386/machdep.c:
sys/amd64/amd64/machdep.c:
	The value reported by FreeBSD as "real memory" when booting
	doesn't match what is later reported by sysctl as hw.realmem.
	This is due to the fact that the value printed during the
	boot process is fetched from smbios data (when possible),
	and accounts for holes in physical memory. On the other
	hand, the value of hw.realmem is unconditionally set to be
	one larger than the highest page of the physical address
	space.

	Fix this by setting hw.realmem to the same value printed
	during boot, this makes hw.realmem honour it's name and
	account properly for physical memory present in the system.

Submitted by:	Roger Pau Monné
Reviewed by:	gibbs
Approved by:	gibbs (mentor)
Approved by:	re (gjb)
MFC 258178:

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)