hw/9pfs: Don't crash when we get a request with not supported 9p operation

Return EOPNOTSUPP as error

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Venkateswararao Jujjuri (JV) <jvrao@linux.vnet.ibm.com>
