vm start: set higher timeout if using PCI passthrough

vm start: set higher timeout if using PCI passthrough

The default VM startup timeout is `max(30, VM memory in GiB)` seconds.
Multiple reports in the forum [0] [1] and the bug tracker [2] suggest
this is too short when using PCI passthrough with a large amount of VM
memory, since QEMU needs to map the whole memory during startup (see
comment #2 in [2]). As a result, VM startup fails with "got timeout".

To work around this, set a larger default timeout if at least one PCI
device is passed through. The question remains how to choose an
appropriate timeout. Users reported the following startup times:

ref | RAM | time  | ratio (s/GiB)
---------------------------------
[1] | 60G |  135s |  2.25
[1] | 70G |  157s |  2.24
[1] | 80G |  277s |  3.46
[2] | 65G |  213s |  3.28
[2] | 96G | >290s | >3.02

The data does not really indicate any simple (e.g. linear)
relationship between RAM and startup time (even data from the same
source). However, to keep the heuristic simple, assume linear growth
and multiply the default timeout by 4 if at least one `hostpci[n]`
option is present, obtaining `4 * max(30, VM memory in GiB)`. This
covers all cases above, and should still leave some headroom.

[0]: https://forum.proxmox.com/threads/83765/post-552071
[1]: https://forum.proxmox.com/threads/126398/post-592826
[2]: https://bugzilla.proxmox.com/show_bug.cgi?id=3502

Suggested-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Friedrich Weber <f.weber@proxmox.com>