exec: Don't abort when we can't allocate guest memory

We abort() on memory allocation failure.  abort() is appropriate for
programming errors.  Maybe most memory allocation failures are
programming errors, maybe not.  But guest memory allocation failure
isn't, and aborting when the user asks for more memory than we can
provide is not nice.  exit(1) instead, and do it in just one place, so
the error message is consistent.

Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-id: 1375276272-15988-8-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <anthony@codemonkey.ws>