I had the exact same problem with Solaris-10-u11-x86.
It looks like GRUB is not installed or installed incorrectly. I was able to fix it by booting from the installation ISO in single user,
looking up the disk name with format and then doing the following (note that the disk name might be different for you):
mount /dev/dsk/c1d0s2 /a
installgrub -m stage1 stage2 /dev/rdsk/c1d0s0
This solution was taken from http://permalink.gmane.org/gmane.os.solaris.gnusolaris.general/170
What's strange is that I was able to install Solaris form the same ISO under Virtual Box with no problems. Xen bug?