VM: Apply mount options to hotplugged disks

[hamistao]

Closes #13677

Related CI tests in: canonical/lxd-ci#403

[hamistao]

Two possible improvements for the future:

• Also supporting passing mount options to the lxd-agent through raw.mount.options, like we already do with containers, which should be easily doable but would require some discussion first, so I did not want to rush it.
• Removing old mount entries when removing devices, when removing/updating a disk device, old mount entries might linger. In the following example, I created a dir share device called foo with readonly enabled, then disabled readonly and enabled it once again, resulting in:

$ lxc exec vm -- mount | grep foo
lxd_foo on /mnt type virtiofs (ro,relatime)
lxd_foo on /mnt type virtiofs (rw,relatime)
lxd_foo on /mnt type virtiofs (ro,relatime)

[hamistao]

@tomponline this should be ready to merge

[tomponline]

Thanks!