libxl: retry QMP PCI device_add

libxl: retry QMP PCI device_add

PCI device assignment to an HVM with stubdom is potentially racy.  First
the PCI device is assigned to the stubdom via the PV PCI protocol.  Then
QEMU is sent a QMP command to attach the PCI device to QEMU running
within the stubdom.  However, the sysfs entries within the stubdom may
not have appeared by the time QEMU receives the device_add command
resulting in errors like:

libxl_qmp.c:1838:qmp_ev_parse_error_messages:Domain 10:Could not open '/sys/bus/pci/devices/0000:00:1f.3/config': No such file or directory

This patch retries the device assignment up to 10 times with a 1 second
delay between.  That roughly matches the overall hotplug timeout for
pci_add_timeout.  pci_add_timeout's initialization is moved to
do_pci_add since retries call into pci_add_qmp_device_add again.

The qmp_ev_parse_error_messages error is still printed since it happens
at a lower level than the pci code controlling the retries.  With that,
the "Retrying PCI add %d" message is also printed at ERROR level to
clarify what is happening.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>