libxl: use correct command line for arm guests.

libxl: use correct command line for arm guests.

We need to use libxl__domain_build_state.pv_cmdline in order to pickup
the correct args when using pygrub. libxl_domain_build_info.cmdline is
any args statically configured by the user.

This is consistent with the call to xc_domain_allocate, which takes
the cmdline too (in that case for x86/PV usage).

state->pv_cmdline is also set for non-pygrub guests, since
libxl__bootloader_run propagates info->cmdline if no bootloader is
configured.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>