[IA64] Allow guest to set the address of shared_info.

[IA64] Allow guest to set the address of shared_info.

Add a new hypercall: SET_SHARED_INFO_VA.
Cleanup of asm-xsi-offsets: do not define absolute address, use a new macro.
Cleanup of linux asm-offsets: use a macro for xen mapped regs.
xensetup.S: set the shared_info address (disabled if using compatibility).
privop.h: May redefined XSI_BASE (not yet enabled for compatibility).
Vocabulary coherence: use XMAPPEDREGS_ prefix.
Cleanup of xensystem.h

Signed-off-by: Tristan Gingold <tristan.gingold@bull.net>