python/machine: raise VMLaunchFailure exception from launch()

This allows us to pack in some extra information about the failure,
which guarantees that if the caller did not *intentionally* cause a
failure (by capturing this Exception), some pretty good clues will be
printed at the bottom of the traceback information.

This will help make failures in the event of a non-negative return code
more obvious when they go unhandled; the current behavior in
_post_shutdown() is to print a warning message only in the event of
signal-based terminations (for negative return codes).

(Note: In Python, catching BaseException instead of Exception catches a
broader array of Exception events, including SystemExit and
KeyboardInterrupt. We do not want to "wrap" such exceptions as a
VMLaunchFailure, because that will 'downgrade' the exception from a
BaseException to a regular Exception. We do, however, want to perform
cleanup in either case, so catch on the broadest scope and
wrap-and-re-raise only in the more targeted scope.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20220201041134.1237016-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

tests/qemu-iotests/tests/mirror-top-perms

@@ -21,7 +21,6 @@
 
 import os
 
-from qemu.aqmp import ConnectError
 from qemu.machine import machine
 from qemu.qmp import QMPConnectError
 
@@ -107,7 +106,7 @@ class TestMirrorTopPerms(iotests.QMPTestCase):
                 self.vm_b.launch()
                 print('ERROR: VM B launched successfully, '
                       'this should not have happened')
-        except (QMPConnectError, ConnectError):
+        except (QMPConnectError, machine.VMLaunchFailure):
             assert 'Is another process using the image' in self.vm_b.get_log()
 
         result = self.vm.qmp('block-job-cancel',