x86/platform: Control warm reset setup via legacy feature flag

Allow to turn off the setup of BIOS-managed warm reset via a new flag in
x86_legacy_features. Besides the UV1, the upcoming jailhose guest support
needs this switched off.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: jailhouse-dev@googlegroups.com
Link: https://lkml.kernel.org/r/44376558129d70a2c1527959811371ef4b82e829.1511770314.git.jan.kiszka@siemens.com