exec: Fix Xen RAM allocation with unusual options

Issues:

* We try to obey -mem-path even though it can't work with Xen.

* To implement -machine mem-merge, we call
  memory_try_enable_merging(new_block->host, size).  But with Xen,
  new_block->host remains null.  Oops.

Fix by separating Xen allocation from normal allocation.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 1375276272-15988-2-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <anthony@codemonkey.ws>
