Since the timers are defined to cover all possible config cases for
several different hypervisors, many of these possibilities generate an
error on qemu. Here is what is currently supported:

RTC: If the -rtc commandline option is available, allow setting
"clock=host"
     or "clock=vm" based on the rtc timer clock='host|guest' value. Also
     add "driftfix=slew" if the tickpolicy is 'catchup', or add nothing
if
     tickpolicy is 'delay'. (Other tickpolicies will raise an error).

     If -rtc isn't available, but -rtc-td-hack is, add that option
     if the tickpolicy is 'catchup', add -rtc-td-hack, if it is 'delay'
     add nothing, and if it's anything else, raise an error.

PIT: If -no-kvm-pit-reinjection is available, and tickpolicy is
     'delay', add that option. if tickpolicy is 'catchup', do
     nothing. Anything else --> raise an error.

     If -no-kvm-pit-reinjection *isn't* available, but -tdf is, when
     tickpolicy is 'catchup' add -tdf. If it's 'delay', do
     nothing. Anything else --> raise an error.

     If neither of those commandline options is available, and
     tickpolicy is anything other than 'delay' (or unspecified), raise
     an error.

HPET: If -no-hpet flag is available and present='no', add -no-hpet.
      If -no-hpet is not available, and present='yes', raise an error.
      If present is unspecified, the default is to do whatever this
      particular qemu does by default, so don't raise an error.

All other timer types are unsupported by QEMU, so they will raise an
error.

* src/qemu/qemu_conf.c: extend qemuBuildClockArgStr() to generate the
  command line arguments for the new options