hypervisor: cleanup ResetType parameter handling

Change ResetType to std::string so readJsonAction() automatically
returns ActionParameterMissing when the field is absent, removing
the redundant explicit check. Also replace propertyValueNotInList
with actionParameterValueNotInList for unsupported ResetType values,
consistent with the Redfish schema semantics for action parameters.

Tested:
- No functionality changes, updated the validation fix.
- Verified property missing error by testing locally in AST2600
platform.
- Service Validator Passed.
+--------------+--------------+--------------+--------------+
|     PASS     |     WARN     |     FAIL     |  NOT TESTED  |
+--------------+--------------+--------------+--------------+
|     5820     |     353      |      0       |     4906     |
+--------------+--------------+--------------+--------------+


Change-Id: I286afe08a66f0fc07c518cd5b227e5312fe5a524
Signed-off-by: Yuvakumar Selvamani <yuvakumars@ami.com>