monitor: Use trad. command interface for HMP pcie_aer_inject_error

All QMP commands use the "new" handler interface (mhandler.cmd_new).
Most HMP commands still use the traditional interface (mhandler.cmd),
but a few use the "new" one.  Complicates handle_user_command() for no
gain, so I'm converting these to the traditional interface.

pcie_aer_inject_error's implementation is split into the
hmp_pcie_aer_inject_error() and pcie_aer_inject_error_print().  The
former is a peculiar crossbreed between HMP and QMP handler.  On
success, it works like a QMP handler: store QDict through ret_data
parameter, return 0.  Printing the QDict is left to
pcie_aer_inject_error_print().  On failure, it works more like an HMP
handler: print error to monitor, return negative number.

To convert to the traditional interface, turn
pcie_aer_inject_error_print() into a command handler wrapping around
hmp_pcie_aer_inject_error().  By convention, this command handler
should be called hmp_pcie_aer_inject_error(), so rename the existing
hmp_pcie_aer_inject_error() to do_pcie_aer_inject_error().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
