libxl: Replace libxl__qmp_initializations by ev_qmp calls

libxl: Replace libxl__qmp_initializations by ev_qmp calls

Setup a timeout of 10s for all the commands. It used to be about 5s
per commands.

The order of command is changed, we call 'query-vnc' before
'change-vnc-password', but that should not matter. That makes it
easier to call 'change-vnc-password' conditionally.

Also 'change' command is replaced by 'change-vnc-password'
because 'change' is deprecated. The new command is available in all
QEMU versions that also have Xen support.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>