KVM: selftests: Refactor UAPI tests into dedicated function

KVM: selftests: Refactor UAPI tests into dedicated function

Move steal time UAPI tests from steal_time_init() into a separate
check_steal_time_uapi() function for better code organization and
maintainability.

Previously, x86 and ARM64 architectures performed UAPI validation
tests within steal_time_init(), mixing initialization logic with
uapi tests.

Changes by architecture:
x86_64:
  - Extract MSR reserved bits test from steal_time_init()
  - Move to check_steal_time_uapi() which tests that setting
    MSR_KVM_STEAL_TIME with KVM_STEAL_RESERVED_MASK fails
ARM64:
  - Extract three UAPI tests from steal_time_init():
     Device attribute support check
     Misaligned IPA rejection (EINVAL)
     Duplicate IPA setting rejection (EEXIST)
  - Move all tests to check_steal_time_uapi()
RISC-V:
  - Add empty check_steal_time_uapi() stub for future use
  - No changes to steal_time_init() (had no tests to extract)

The new check_steal_time_uapi() function:
  - Is called once before the per-VCPU test loop

No functional change intended.

Suggested-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Signed-off-by: Jiakai Xu <xujiakai2025@iscas.ac.cn>
Signed-off-by: Jiakai Xu <jiakaiPeanut@gmail.com>
Reviewed-by: Andrew Jones <andrew.jones@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260303010859.1763177-3-xujiakai2025@iscas.ac.cn
Signed-off-by: Anup Patel <anup@brainfault.org>