x86/hvm: remove stray lock release from hvm_ioreq_server_init()

If HVM_PARAM_IOREQ_PFN, HVM_PARAM_BUFIOREQ_PFN, or HVM_PARAM_BUFIOREQ_EVTCHN
parameters are read when guest domain is dying it leads to the following
ASSERT:

(XEN) Assertion '_raw_spin_is_locked(lock)' failed at ...workspace/KERNEL/xen/xen/include/asm/spinlock.h:18
(XEN) ----[ Xen-4.5-unstable  x86_64  debug=y  Not tainted ]----
...
(XEN) Xen call trace:
(XEN)    [<ffff82d08012b07f>] _spin_unlock+0x27/0x30
(XEN)    [<ffff82d0801b6103>] hvm_create_ioreq_server+0x3df/0x49a
(XEN)    [<ffff82d0801bcceb>] do_hvm_op+0x12bf/0x27a0
(XEN)    [<ffff82d08022b9bb>] syscall_enter+0xeb/0x145

The root cause of this issue is the fact that ioreq_server.lock is being
released twice - first in hvm_ioreq_server_init() and then in hvm_create_ioreq_server().
Drop the lock release from hvm_ioreq_server_init() as we don't take it here, do minor
label cleanup.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>