libs/guest: Move the guest ABI check earlier into xc_dom_parse_image()

libs/guest: Move the guest ABI check earlier into xc_dom_parse_image()

Xen may not support 32-bit PV guest for a number of reasons (lack of
CONFIG_PV32, explicit pv=no-32 command line argument, or implicitly
due to CET being enabled) and advertises this to the toolstack via the
absence of xen-3.0-x86_32p ABI.

Currently, when trying to boot a 32-bit PV guest, the ABI check is too
late and the build explodes in the following manner yielding an
unhelpful error message:

  xc: error: panic: xg_dom_boot.c:121: xc_dom_boot_mem_init: can't allocate low memory for domain: Out of memory
  libxl: error: libxl_dom.c:586:libxl__build_dom: xc_dom_boot_mem_init failed: Operation not supported
  libxl: error: libxl_create.c:1573:domcreate_rebuild_done: Domain 1:cannot (re-)build domain: -3
  libxl: error: libxl_domain.c:1182:libxl__destroy_domid: Domain 1:Non-existant domain
  libxl: error: libxl_domain.c:1136:domain_destroy_callback: Domain 1:Unable to destroy guest
  libxl: error: libxl_domain.c:1063:domain_destroy_cb: Domain 1:Destruction of domain failed

Move the ABI check earlier into xc_dom_parse_image() along with other
ELF-note feature checks.  With this adjustment, it now looks like
this:

  xc: error: panic: xg_dom_boot.c:88: xc_dom_compat_check: guest type xen-3.0-x86_32p not supported by xen kernel, sorry: Invalid kernel
  libxl: error: libxl_dom.c:571:libxl__build_dom: xc_dom_parse_image failed
  domainbuilder: detail: xc_dom_release: called
  libxl: error: libxl_create.c:1573:domcreate_rebuild_done: Domain 11:cannot (re-)build domain: -3
  libxl: error: libxl_domain.c:1182:libxl__destroy_domid: Domain 11:Non-existant domain
  libxl: error: libxl_domain.c:1136:domain_destroy_callback: Domain 11:Unable to destroy guest
  libxl: error: libxl_domain.c:1063:domain_destroy_cb: Domain 11:Destruction of domain failed

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jane Malalane <jane.malalane@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <iwj@xenproject.org>