Damn, this affects me too. I upgraded from 5.0 to 5.1 with apt-get dist-upgrade, and afterwards it times out on boot waiting for "dev-pve-swap.device" to become ready, then hangs forever waiting for LVM2 activation ("A start job is running for Activation of LVM2 logical volumes").
I erased my Proxmox SSD, disconnected all my ZFS hard disks and did a clean install of 5.1 from the 3rd ISO. This booted fine. However, when I connect my ZFS disks back up, it starts hanging on boot again waiting for dev-pve-swap (disconnecting them allows it to keep working). Booting in single-user mode with init=/bin/bash works fine, and I can import the zpool from the disks without any errors. These tested kernels triggered the issue:
pve-kernel-4.13.8-3-pve
pve-kernel-4.13.8-2-pve
pve-kernel-4.13.4-1-pve
4.10.17-5-pve works perfectly.
I'm booting UEFI and my root disk is formatted with LVM using the default options in the installer.