x86/pvm: Disable some unsupported syscalls and features

n the PVM guest, the LDT won't be loaded into hardware, rendering it
ineffective. Consequently, the modify_ldt() syscall should be disabled.
Additionally, the VSYSCALL address is not within the allowed address
range, making full emulation of the vsyscall page unsupported in the PVM
guest. It is recommended to use XONLY mode instead. Furthermore,
SYSENTER (Intel) and SYSCALL32 (AMD) are not supported by the
hypervisor, so they should not be used in VDSO.

Suggested-by: Lai Jiangshan <[email protected]>
Signed-off-by: Hou Wenlong <[email protected]>