Today I created a new guest machine, and migration works for it! So I compared xml files from old and new guests and noticed the difference
<type arch='x86_64' machine='pc-0.12'>hvm</type> (old guests) vs. <type arch='x86_64' machine='pc-1.0'>hvm</type> (new guest)
I changed all guest configs to pc-1.0 and now migration is working again.
Today I created a new guest machine, and migration works for it! So I compared xml files from old and new guests and noticed the difference
<type arch='x86_64' machine= 'pc-0.12' >hvm</type> (old guests) 'pc-1.0' >hvm</type> (new guest)
vs.
<type arch='x86_64' machine=
I changed all guest configs to pc-1.0 and now migration is working again.