OvmfPkg/XenBusDxe: eliminate AsciiStrCpy() calls

OvmfPkg/XenBusDxe: eliminate AsciiStrCpy() calls

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

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>