x86: Fix memset() arguments order

vmcb_setup() had them wrong, so VMCB was not zeroed correctly.
x86: Enable Extended Interrupt LVT

Extended Interrupt LVTs are vendor-specific APIC registers (0x50-0x53),
defined by AMD (see APMv2, Sect. 16.3.2). Jailhouse lists this MMIO
range as reserved, as it is not used on Intel systems. On AMD systems,
Extended Interrupt LVT is no more "privileged" than other LVT registers,
so we enable them if Jailhouse is built for AMD.
x86: Fix GCC 4.8 warning

It is reported that GCC 4.8 is unable to determine that 'reg' variable
in svm_handle_cr() is actually always defined (if x86_parse_mov_to_cr() fails,
the code that uses 'reg' is not executed).

Workaround this by setting reg to a needlessly high value.
x86: Remove unused defines

X86_INST_LEN_VMM?CALL are never referenced in the code, so remove
the defines altogether.