x86/traps: factor out pv_trap_init

x86/traps: factor out pv_trap_init

Factor out pv_trap_init and call it at the beginning of trap_init. We
then need to tune the code to generate stub handlers in entry.S. Take
the chance to tune init_irq_data so that 0x80 and 0x82 can be used for
regular interrupts in !CONFIG_PV case.

While at it, fix some coding style issues in init_irq_data and replace
0x80 with LEGACY_SYSCALL_VECTOR in pv_trap_init.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reivewed-by: Andrew Cooper <andrew.cooper3@citrix.com>