xen: fix booting ballooned down hvm guest

Commit 96edd61d ("xen/balloon: don't
online new memory initially") introduced a regression when booting a
HVM domain with memory less than mem-max: instead of ballooning down
immediately the system would try to use the memory up to mem-max
resulting in Xen crashing the domain.

For HVM domains the current size will be reflected in Xenstore node
memory/static-max instead of memory/target.

Additionally we have to trigger the ballooning process at once.

Cc: <stable@vger.kernel.org> # 4.13
Fixes: 96edd61d ("xen/balloon: don't
       online new memory initially")

Reported-by: Simon Gaiser <hw42@ipsumj.de>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>