KVM: arm64: selftests: Handle feature fields with nonzero minimum value correctly

KVM: arm64: selftests: Handle feature fields with nonzero minimum value correctly

There are some feature fields with nonzero minimum valid value. Make
sure get_safe_value() won't return invalid field values for them.
Also fix a bug that wrongly uses the feature bits type as the feature
bits sign causing all fields as signed in the get_safe_value() and
get_invalid_value().

Fixes: 54a9ea73527d ("KVM: arm64: selftests: Test for setting ID register from usersapce")
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Reported-by: Itaru Kitayama <itaru.kitayama@linux.dev>
Tested-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
Signed-off-by: Jing Zhang <jingzhangos@google.com>
Link: https://lore.kernel.org/r/20240115220210.3966064-2-jingzhangos@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>