xen-hvm.c: Always return -1 when failure occurs in xen_hvm_init()

When failure occurs, it need to use "return -1" instead of exit(1), so
an upper layer has a chance to print failure information, too.

For simplicity, in xen_hvm_init(), also use '-1' instead of all
'-errno', since all related upper callers always exit(1) on failure.

It is not a normal function, it does not release related resources when
return -1, so need give related comments for it.

It passes common check:

  "./configure --enable-xen && make && make check"
  "echo $? == 0"

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
