Description of problem:
Error msg should be improved when the start time is out of range or malformed

Version-Release number of selected component (if applicable):
libvirt-8.5.0-6.el9.x86_64

How reproducible:
100%

Steps to Reproduce:
Scenario 1:
1. Define a guest with following xml
 <clock offset='absolute' start='4294967296'>
    <timer name='rtc' tickpolicy='catchup'/>
    <timer name='pit' tickpolicy='delay'/>
    <timer name='hpet' present='no'/>
  </clock>

2. define the guest
# virsh define avocado-vt-vm1.xml 
Domain 'avocado-vt-vm1' defined from avocado-vt-vm1.xml

3. start the guest
# virsh start avocado-vt-vm1 
Domain 'avocado-vt-vm1' started

4. check qemu cmdline
...
-rtc base=2106-02-07T06:28:16,driftfix=slew -global kvm-pit.lost_tick_policy=delay -no-hpet
...

5.Edit the guest xml, should make sure the start time is not 4294967296 before edit
# virsh edit avocado-vt-vm1
  <clock offset='absolute' start='4294967296'>    <== change the start to 4294967296
error: XML document failed to validate against schema: Unable to validate doc against /usr/share/libvirt/schemas/domain.rng
Expecting an element timer, got nothing
Extra element clock in interleave
Element domain failed to validate content

Failed. Try again? [y,n,i,f,?]: 

From the code,
        /* absolute clock start time for VIR_DOMAIN_CLOCK_OFFSET_ABSOLUTE */
        unsigned long long starttime;
The data type of starttime is unsigned long long

          <group>
            <attribute name="offset">
              <value>absolute</value>
            </attribute>
            <attribute name="start">
              <ref name="unsignedInt"/>
            </attribute>
          </group>

But data type in rng file is unsignedInt

Scenario2:
1. define a guest with start time large than 253402300799
  <clock offset='absolute' start='253402300800'>
    <timer name='rtc' tickpolicy='catchup'/>
    <timer name='pit' tickpolicy='delay'/>
    <timer name='hpet' present='no'/>
  </clock>

2. start the guest
virsh start avocado-vt-vm1
error: Failed to start domain 'avocado-vt-vm1'
error: internal error: process exited while connecting to monitor: 2022-09-22T08:47:48.299885Z qemu-kvm: invalid datetime format
valid formats: '2006-06-17T16:01:21' or '2006-06-17'

(qemu will report the error to show the start time is out of range, and the error msg is misleading here, since it is said in libvirt doc: start attribute takes an epoch timestamp)



Actual results:
The data type of start time is not self-consistent.

Expected results:


Additional info: