OvmfPkg/XenBusDxe: eliminate AsciiStrCat() calls

OvmfPkg/XenBusDxe: eliminate AsciiStrCat() calls

AsciiStrCat() is deprecated / disabled under the
DISABLE_NEW_DEPRECATED_INTERFACES feature test macro.

Replace AsciiStrCat() with AsciiSPrint(). Spell out the (already existent)
PrintLib dependency in the INF file. Add an explicit ASSERT() to document
that XenStoreJoin() assumes that the pool allocation always succeeds.

Cc: Anthony PERARD <anthony.perard@citrix.com>
Cc: Gary Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=166
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>