CA-120586: Convert Failed_to_start_emulator into a proper API exception

[johnelse]

Before:

# xe vm-start uuid=76af1940-4fe9-4a09-f384-64188ebac3e5 
The server failed to handle your request, due to an internal error.  The given message may give details useful for debugging the problem.
message: xenopsd internal error: Device.Ioemu_failed("vgpu exited unexpectedly")

After:

# xe vm-start uuid=76af1940-4fe9-4a09-f384-64188ebac3e5 
An emulator required to run this VM failed to start
vm: 76af1940-4fe9-4a09-f384-64188ebac3e5 (ubuntu-0)
name: vgpu
msg: Daemon exited unexpectedly

[johnelse]

Depends on xapi-project/xcp-idl#111

[simonjbeaumont]

This patch is good. Blocked on the update of xapi-project/xenopsd#254

[johnelse]

Unblocked now.